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/chrome/chrome_browser_host_impl.h

@@ -38,6 +38,11 @@ class ChromeBrowserHostImpl : public CefBrowserHostBase {
   static CefRefPtr<ChromeBrowserHostImpl> Create(
       const CefBrowserCreateParams& params);
 
+  // Safe (checked) conversion from CefBrowserHostBase to ChromeBrowserHostImpl.
+  // Use this method instead of static_cast.
+  static CefRefPtr<ChromeBrowserHostImpl> FromBaseChecked(
+      CefRefPtr<CefBrowserHostBase> host_base);
+
   // Returns the browser associated with the specified RenderViewHost.
   static CefRefPtr<ChromeBrowserHostImpl> GetBrowserForHost(
       const content::RenderViewHost* host);
@@ -62,6 +67,10 @@ class ChromeBrowserHostImpl : public CefBrowserHostBase {
   // CefBrowserHostBase methods called from CefFrameHostImpl:
   void OnSetFocus(cef_focus_source_t source) override;
 
+  // CefBrowserHostBase methods:
+  bool IsWindowless() const override { return false; }
+  bool IsAlloyStyle() const override { return false; }
+
   // CefBrowserHost methods:
   void CloseBrowser(bool force_close) override;
   bool TryCloseBrowser() override;
@@ -72,9 +81,7 @@ class ChromeBrowserHostImpl : public CefBrowserHostBase {
             bool matchCase,
             bool findNext) override;
   void StopFinding(bool clearSelection) override;
-  void CloseDevTools() override;
-  bool HasDevTools() override;
-  bool IsWindowRenderingDisabled() override;
+  bool IsWindowRenderingDisabled() override { return false; }
   void WasResized() override;
   void WasHidden(bool hidden) override;
   void NotifyScreenInfoChanged() override;
@@ -124,8 +131,6 @@ class ChromeBrowserHostImpl : public CefBrowserHostBase {
 
  protected:
   bool Navigate(const content::OpenURLParams& params) override;
-  void ShowDevToolsOnUIThread(
-      std::unique_ptr<CefShowDevToolsParams> params) override;
 
  private:
   friend class ChromeBrowserDelegate;
@@ -147,7 +152,7 @@ class ChromeBrowserHostImpl : public CefBrowserHostBase {
   // created. Must be called on the UI thread.
   void Attach(content::WebContents* web_contents,
               bool is_devtools_popup,
-              CefRefPtr<ChromeBrowserHostImpl> opener);
+              CefRefPtr<CefBrowserHostBase> opener);
 
   // Called from ChromeBrowserDelegate::AddNewContents to take ownership of a
   // popup WebContents. |browser_create_params| may be empty for default Browser
@@ -161,9 +166,6 @@ class ChromeBrowserHostImpl : public CefBrowserHostBase {
   // before the new Browser is added. Must be called on the UI thread.
   void SetBrowser(Browser* browser);
 
-  void SetDevToolsBrowserHost(
-      base::WeakPtr<ChromeBrowserHostImpl> devtools_browser_host);
-
   // CefBrowserHostBase methods:
   void WindowDestroyed() override;
   bool WillBeDestroyed() const override;
@@ -178,8 +180,6 @@ class ChromeBrowserHostImpl : public CefBrowserHostBase {
   Browser* browser_ = nullptr;
   CefWindowHandle host_window_handle_ = kNullWindowHandle;
 
-  base::WeakPtr<ChromeBrowserHostImpl> devtools_browser_host_;
-
   base::WeakPtrFactory<ChromeBrowserHostImpl> weak_ptr_factory_{this};
 };