chrome: Add support for Alloy style browsers and windows (see #3681)

Split the Alloy runtime into bootstrap and style components. Support
creation of Alloy style browsers and windows with the Chrome runtime.
Chrome runtime (`--enable-chrome-runtime`) + Alloy style
(`--use-alloy-style`) supports Views (`--use-views`), native parent
(`--use-native`) and windowless rendering
(`--off-screen-rendering-enabled`).

Print preview is supported in all cases except with windowless rendering
on all platforms and native parent on MacOS. It is disabled by default
with Alloy style for legacy compatibility. Where supported it can be
enabled or disabled globally using `--[enable|disable]-print-preview` or
configured on a per-RequestContext basis using the
`printing.print_preview_disabled` preference. It also behaves as
expected when triggered via the PDF viewer print button.

Chrome runtime + Alloy style behavior differs from Alloy runtime in the
following significant ways:

- Supports Chrome error pages by default.
- DevTools popups are Chrome style only (cannot be windowless).
- The Alloy extension API will not supported.

Chrome runtime + Alloy style passes all expected Alloy ceftests except
the following:

- `DisplayTest.AutoResize` (Alloy extension API not supported)
- `DownloadTest.*` (Download API not yet supported)
- `ExtensionTest.*` (Alloy extension API not supported)

This change also adds Chrome runtime support for
CefContextMenuHandler::RunContextMenu (see #3293).

This change also explicitly blocks (and doesn't retry) FrameAttached
requests from PDF viewer and print preview excluded frames (see #3664).

Known issues specific to Chrome runtime + Alloy style:
- DevTools popup with windowless rendering doesn't load successfully.
  Use windowed rendering or remote debugging as a workaround.
- Chrome style Window with Alloy style BrowserView (`--use-alloy-style
  --use-chrome-style-window`) does not show Chrome theme changes.

To test:
- Run `ceftests --enable-chrome-runtime --use-alloy-style
       [--use-chrome-style-window] [--use-views|--use-native]
       --gtest_filter=...`
- Run `cefclient --enable-chrome-runtime --use-alloy-style
       [--use-chrome-style-window]
       [--use-views|--use-native|--off-screen-rendering-enabled]`
- Run `cefsimple --enable-chrome-runtime --use-alloy-style [--use-views]`

libcef/browser/devtools/devtools_protocol_manager.h

@@ -0,0 +1,56 @@
+// Copyright (c) 2020 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_DEVTOOLS_DEVTOOLS_PROTOCOL_MANAGER_H_
+#define CEF_LIBCEF_BROWSER_DEVTOOLS_DEVTOOLS_PROTOCOL_MANAGER_H_
+#pragma once
+
+#include <memory>
+
+#include "include/cef_browser.h"
+
+class CefBrowserHostBase;
+class CefDevToolsController;
+
+namespace content {
+class WebContents;
+}
+
+// Manages DevTools protocol messages without an active frontend. Methods must
+// be called on the UI thread unless otherwise indicated.
+class CefDevToolsProtocolManager {
+ public:
+  // |inspected_browser| will outlive this object.
+  explicit CefDevToolsProtocolManager(CefBrowserHostBase* inspected_browser);
+
+  CefDevToolsProtocolManager(const CefDevToolsProtocolManager&) = delete;
+  CefDevToolsProtocolManager& operator=(const CefDevToolsProtocolManager&) =
+      delete;
+
+  ~CefDevToolsProtocolManager();
+
+  // See CefBrowserHost methods of the same name for documentation.
+  bool SendDevToolsMessage(const void* message, size_t message_size);
+  int ExecuteDevToolsMethod(int message_id,
+                            const CefString& method,
+                            CefRefPtr<CefDictionaryValue> param);
+
+  // These methods are used to implement
+  // CefBrowserHost::AddDevToolsMessageObserver. CreateRegistration is safe to
+  // call on any thread. InitializeRegistrationOnUIThread should be called
+  // immediately afterwards on the UI thread.
+  static CefRefPtr<CefRegistration> CreateRegistration(
+      CefRefPtr<CefDevToolsMessageObserver> observer);
+  void InitializeRegistrationOnUIThread(
+      CefRefPtr<CefRegistration> registration);
+
+ private:
+  bool EnsureController();
+
+  CefBrowserHostBase* const inspected_browser_;
+
+  std::unique_ptr<CefDevToolsController> devtools_controller_;
+};
+
+#endif  // CEF_LIBCEF_BROWSER_DEVTOOLS_DEVTOOLS_PROTOCOL_MANAGER_H_