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]`

tests/cefclient/browser/client_handler.h

@@ -42,6 +42,13 @@ class ClientHandler : public BaseClientHandler,
   // otherwise indicated.
   class Delegate {
    public:
+    // Returns true if the window should use Views. Safe to call on any thread.
+    virtual bool UseViews() const = 0;
+
+    // Returns true if the window should use Alloy style. Safe to call on any
+    // thread.
+    virtual bool UseAlloyStyle() const = 0;
+
     // Called when the browser is created.
     virtual void OnBrowserCreated(CefRefPtr<CefBrowser> browser) = 0;
 
@@ -354,6 +361,12 @@ class ClientHandler : public BaseClientHandler,
   // THREAD SAFE MEMBERS
   // The following members may be accessed from any thread.
 
+  // True if this handler uses Views.
+  const bool use_views_;
+
+  // True if this handler uses Alloy style.
+  const bool use_alloy_style_;
+
   // True if this handler uses off-screen rendering.
   const bool is_osr_;