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/main_context.h

@@ -43,18 +43,16 @@ class MainContext {
   // Returns the background color.
   virtual cef_color_t GetBackgroundColor() = 0;
 
-  // Returns true if the Chrome runtime will be used.
-  virtual bool UseChromeRuntime() = 0;
+  // Returns true if the Chrome runtime bootstrap will be used.
+  virtual bool UseChromeBootstrap() = 0;
 
-  // Returns true if a native parent window is being used with the Chrome
-  // runtime.
-  virtual bool UseChromeRuntimeNative() = 0;
+  // Returns true if the Views framework will be used as the global default.
+  virtual bool UseViewsGlobal() = 0;
 
-  // Returns true if the Views framework will be used.
-  virtual bool UseViews() = 0;
-
-  // Returns true if windowless (off-screen) rendering will be used.
-  virtual bool UseWindowlessRendering() = 0;
+  // Returns true if Alloy style will be used as the global default. Alloy style
+  // is always used with the Alloy runtime bootstrap and optional with the
+  // Chrome runtime bootstrap. Windowless rendering requires Alloy style.
+  virtual bool UseAlloyStyleGlobal() = 0;
 
   // Returns true if touch events are enabled.
   virtual bool TouchEventsEnabled() = 0;