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/alloy/alloy_browser_host_impl.h

@@ -62,6 +62,11 @@ class AlloyBrowserHostImpl : public CefBrowserHostBase,
   static CefRefPtr<AlloyBrowserHostImpl> Create(
       CefBrowserCreateParams& create_params);
 
+  // Safe (checked) conversion from CefBrowserHostBase to AlloyBrowserHostImpl.
+  // Use this method instead of static_cast.
+  static CefRefPtr<AlloyBrowserHostImpl> FromBaseChecked(
+      CefRefPtr<CefBrowserHostBase> host_base);
+
   // Returns the browser associated with the specified RenderViewHost.
   static CefRefPtr<AlloyBrowserHostImpl> GetBrowserForHost(
       const content::RenderViewHost* host);
@@ -85,8 +90,6 @@ class AlloyBrowserHostImpl : public CefBrowserHostBase,
             bool matchCase,
             bool findNext) override;
   void StopFinding(bool clearSelection) override;
-  void CloseDevTools() override;
-  bool HasDevTools() override;
   bool IsWindowRenderingDisabled() override;
   void WasResized() override;
   void WasHidden(bool hidden) override;
@@ -126,9 +129,9 @@ class AlloyBrowserHostImpl : public CefBrowserHostBase,
   void ExecuteChromeCommand(int command_id,
                             cef_window_open_disposition_t disposition) override;
 
-  // Returns true if windowless rendering is enabled.
+  // CefBrowserHostBase methods:
   bool IsWindowless() const override;
-
+  bool IsAlloyStyle() const override { return true; }
   bool IsVisible() const override;
 
   // Returns true if this browser supports picture-in-picture.
@@ -147,7 +150,6 @@ class AlloyBrowserHostImpl : public CefBrowserHostBase,
   void CancelContextMenu();
 
   bool MaybeAllowNavigation(content::RenderFrameHost* opener,
-                            bool is_guest_view,
                             const content::OpenURLParams& params) override;
 
   // Convert from view DIP coordinates to screen coordinates. If
@@ -290,10 +292,6 @@ class AlloyBrowserHostImpl : public CefBrowserHostBase,
       override;
   void WebContentsDestroyed() override;
 
- protected:
-  void ShowDevToolsOnUIThread(
-      std::unique_ptr<CefShowDevToolsParams> params) override;
-
  private:
   friend class CefBrowserPlatformDelegateAlloy;