chrome: Add Views API integration (see issue #2969)

The Chrome browser can now be hosted in a Views-based application on Windows
and Linux.

To launch a fully-featured Chrome window using cefsimple:
$ cefsimple --enable-chrome-runtime

To launch a minimally-styled Views-hosted window using cefsimple:
$ cefsimple --enable-chrome-runtime --use-views

To launch a fully-styled Views-hosted window using cefclient:
$ cefclient --enable-chrome-runtime --use-views

Views unit tests also now pass with the Chrome runtime enabled:
$ ceftests --gtest_filter=Views* --enable-chrome-runtime

Known issues:
- Popup browsers cannot be intercepted and reparented.

libcef/browser/chrome/views/chrome_browser_frame.h

@@ -0,0 +1,109 @@
+// Copyright 2021 The Chromium Embedded Framework Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be found
+// in the LICENSE file.
+
+#ifndef CEF_LIBCEF_BROWSER_CHROME_VIEWS_CHROME_BROWSER_FRAME_H_
+#define CEF_LIBCEF_BROWSER_CHROME_VIEWS_CHROME_BROWSER_FRAME_H_
+#pragma once
+
+#include "chrome/browser/ui/browser.h"
+#include "chrome/browser/ui/views/frame/browser_frame.h"
+
+// An overview of the Chrome Browser object model is provided below. Object
+// creation normally begins with a call to Browser::Create(CreateParams) which
+// then creates the necessary Browser view, window and frame objects. CEF has
+// modified the default object creation model are described below to better
+// integrate with the existing CEF Views APIs.
+//
+// OVERVIEW
+//
+// Browser and BrowserWindow are the primary Chrome objects. Browser provides
+// the concrete state and mutation methods while BrowserWindow is an interface
+// implemented by the platform-specific "view" of the Browser window.
+//
+// Browser:
+// - Creates a BrowserView (aka BrowserWindow) and BrowserFrame (aka Widget) via
+//   a call to BrowserWindow::CreateBrowserWindow() in the Browser constructor.
+//   - An existing BrowserWindow can alternately be specified via the
+//     Browser::CreateParams::window parameter.
+// - Owned by the BrowserView after creation.
+//
+// The Chrome Views implementation uses BrowserView to represent the browser
+// client area and BrowserFrame (plus helpers) to represent the non-client
+// window frame.
+//
+// BrowserView:
+// - Extends BrowserWindow, views::ClientView, views::WidgetDelegate.
+// - Owns the Browser.
+// - References the BrowserFrame.
+// - Passed to Widget::Init() via Widget::InitParams::delegate to receive
+//   WidgetDelegate callbacks.
+// - Extended by CEF as ChromeBrowserView.
+// BrowserFrame:
+// - Extends Widget (aka views::internal::NativeWidgetDelegate).
+// - References the BrowserView.
+// - Creates/owns a DesktopBrowserFrameAura (aka NativeBrowserFrame) via
+//   BrowserFrame::InitBrowserFrame().
+// - Extended by CEF as ChromeBrowserFrame.
+//
+// Chrome custom window/frame handling is implemented using platform-specific
+// objects.
+//
+// DesktopBrowserFrameAura:
+// - Extends NativeBrowserFrame, DesktopNativeWidgetAura.
+// - Acts as a helper for BrowserFrame.
+// - Creates/references a BrowserDesktopWindowTreeHostWin via
+//   DesktopBrowserFrameAura::InitNativeWidget().
+// BrowserDesktopWindowTreeHostWin (for Windows):
+// - Extends DesktopWindowTreeHost.
+// - References DesktopBrowserFrameAura, BrowserView, BrowserFrame.
+// - Passed to Widget::Init() via Widget::InitParams::desktop_window_tree_host.
+//
+// CEF MODIFICATIONS
+//
+// The CEF Views integration uses an alternative approach of creating the
+// ChromeBrowserFrame in CefWindowView::CreateWidget() and the
+// ChromeBrowserView in CefBrowserViewImpl::CreateRootView().
+// The object associations described above are then configured via
+// ChromeBrowserView::AddedToWidget() and ChromeBrowserHostImpl::Create()
+// after the BrowserView is added to the Widget. The Chromium code has been
+// patched to allow later initialization of the Browser, BrowserFrame and
+// BrowserView members to support this model.
+//
+// CEF does not use Chrome's NativeBrowserFrame (aka DesktopBrowserFrameAura),
+// BrowserNonClientFrameView or BrowserRootView objects (all normally created by
+// BrowserFrame during Widget initialization). Consequently
+// |BrowserFrame::native_browser_frame_| and |BrowserFrame::browser_frame_view_|
+// (sometimes retrieved via BrowserFrame::GetFrameView) will be nullptr and the
+// Chromium code has been patched to add the necessary null checks.
+//
+// CEF does not pass ChromeBrowserView as the WidgetDelegate when the Widget is
+// initialized in CefWindowView::CreateWidget(). Some of the WidgetDelegate
+// callbacks may need to be routed from CefWindowView to ChromeBrowserView in
+// the future.
+//
+// See the chrome_runtime_views.patch file for the complete set of related
+// modifications.
+
+class BrowserView;
+
+// Widget for a Views-hosted Chrome browser. Created in
+// CefWindowView::CreateWidget() when the Chrome runtime is enabled.
+class ChromeBrowserFrame : public BrowserFrame {
+ public:
+  ChromeBrowserFrame() {}
+  ChromeBrowserFrame(const ChromeBrowserFrame&) = delete;
+  ChromeBrowserFrame& operator=(const ChromeBrowserFrame&) = delete;
+
+  void Init(BrowserView* browser_view, std::unique_ptr<Browser> browser);
+
+  // views::Widget methods:
+  views::internal::RootView* CreateRootView() override;
+  std::unique_ptr<views::NonClientFrameView> CreateNonClientFrameView()
+      override;
+
+ private:
+  Browser* browser_ = nullptr;
+};
+
+#endif  // CEF_LIBCEF_BROWSER_CHROME_VIEWS_CHROME_BROWSER_FRAME_H_