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/views/browser_view_impl.h

@@ -48,7 +48,8 @@ class CefBrowserViewImpl
   // nullptr.
   static CefRefPtr<CefBrowserViewImpl> CreateForPopup(
       const CefBrowserSettings& settings,
-      CefRefPtr<CefBrowserViewDelegate> delegate);
+      CefRefPtr<CefBrowserViewDelegate> delegate,
+      bool is_devtools);
 
   // Called from CefBrowserPlatformDelegate[Chrome]Views.
   void WebContentsCreated(content::WebContents* web_contents);
@@ -65,6 +66,7 @@ class CefBrowserViewImpl
   CefRefPtr<CefBrowser> GetBrowser() override;
   CefRefPtr<CefView> GetChromeToolbar() override;
   void SetPreferAccelerators(bool prefer_accelerators) override;
+  cef_runtime_style_t GetRuntimeStyle() override;
 
   // CefView methods:
   CefRefPtr<CefBrowserView> AsBrowserView() override { return this; }
@@ -77,7 +79,6 @@ class CefBrowserViewImpl
   void GetDebugInfo(base::Value::Dict* info, bool include_children) override;
 
   // CefBrowserViewView::Delegate methods:
-  void OnBrowserViewAdded() override;
   void AddedToWidget() override;
   void RemovedFromWidget() override;
   void OnBoundsChanged() override;
@@ -92,11 +93,15 @@ class CefBrowserViewImpl
   // Return the CefWindowImpl hosting this object.
   CefWindowImpl* cef_window_impl() const;
 
+  bool IsAlloyStyle() const { return is_alloy_style_; }
+  bool IsChromeStyle() const { return !is_alloy_style_; }
+
  private:
   // Create a new implementation object.
   // Always call Initialize() after creation.
   // |delegate| may be nullptr.
-  explicit CefBrowserViewImpl(CefRefPtr<CefBrowserViewDelegate> delegate);
+  CefBrowserViewImpl(CefRefPtr<CefBrowserViewDelegate> delegate,
+                     bool is_devtools_popup);
 
   void SetPendingBrowserCreateParams(
       const CefWindowInfo& window_info,
@@ -122,6 +127,9 @@ class CefBrowserViewImpl
 
   void DisassociateFromWidget();
 
+  // True if the browser is Alloy style, otherwise Chrome style.
+  const bool is_alloy_style_;
+
   std::unique_ptr<CefBrowserCreateParams> pending_browser_create_params_;
 
   CefRefPtr<CefBrowserHostBase> browser_;