views: Add support for OS and Chrome themes (fixes #3610, fixes #3671)

Controls now respect OS and Chrome themes by default for both Alloy
and Chrome runtimes. Chrome themes (mode and colors) can be configured
using the new CefRequestContext::SetChromeColorScheme method. Individual
theme colors can be overridden using the new CefWindowDelegate::
OnThemeColorsChanged and CefWindow::SetThemeColor methods.

The `--force-light-mode` and `--force-dark-mode` command-line flags are
now respected on all platforms as an override for the OS theme.

The current Chrome theme, if any, will take precedence over the OS theme
when determining light/dark status. On Windows and MacOS the titlebar
color will also be updated to match the light/dark theme.

Testable as follows:
- Run: `cefclient --enable-chrome-runtime` OR
       `cefclient --use-views --persist-user-preferences --cache-path=...`
  - App launches with default OS light/dark theme colors.
  - Change OS dark/light theme under system settings. Notice that theme
    colors change as expected.
  - Right click, select items from the new Theme sub-menu. Notice that
    theme colors behave as expected.
  - Exit and relaunch the app. Notice that the last-used theme colors are
    applied on app restart.
- Add `--background-color=green` to above command-line.
  - Perform the same actions as above. Notice that all controls start
    and remain green throughout (except some icons with Chrome runtime).
- Add `--force-light-mode` or `--force-dark-mode` to above command-line.
  - Perform the same actions as above. Notice that OS dark/light theme
    changes are ignored, but Chrome theme changes work as expected.

include/views/cef_window_delegate.h

@@ -164,12 +164,11 @@ class CefWindowDelegate : public CefPanelDelegate {
   }
 
   ///
-  /// Return whether the titlebar height should be overridden,
-  /// and sets the height of the titlebar in |titlebar_height|.
-  /// On macOS, it can also be used to adjust the vertical position
-  /// of the traffic light buttons in frameless windows.
-  /// The buttons will be positioned halfway down the titlebar
-  /// at a height of |titlebar_height| / 2.
+  /// Return whether the titlebar height should be overridden, and sets the
+  /// height of the titlebar in |titlebar_height|. On macOS, it can also be used
+  /// to adjust the vertical position of the traffic light buttons in frameless
+  /// windows. The buttons will be positioned halfway down the titlebar at a
+  /// height of |titlebar_height| / 2.
   ///
   /*--cef()--*/
   virtual bool GetTitlebarHeight(CefRefPtr<CefWindow> window,
@@ -222,6 +221,41 @@ class CefWindowDelegate : public CefPanelDelegate {
                           const CefKeyEvent& event) {
     return false;
   }
+
+  ///
+  /// Called after the native/OS or Chrome theme for |window| has changed.
+  /// |chrome_theme| will be true if the notification is for a Chrome theme.
+  ///
+  /// Native/OS theme colors are configured globally and do not need to be
+  /// customized for each Window individually. An example of a native/OS theme
+  /// change that triggers this callback is when the user switches between dark
+  /// and light mode during application lifespan. Native/OS theme changes can be
+  /// disabled by passing the `--force-dark-mode` or `--force-light-mode`
+  /// command-line flag.
+  ///
+  /// Chrome theme colors will be applied and this callback will be triggered
+  /// if/when a BrowserView is added to the Window's component hierarchy. Chrome
+  /// theme colors can be configured on a per-RequestContext basis using
+  /// CefRequestContext::SetChromeColorScheme or (Chrome runtime only) by
+  /// visiting chrome://settings/manageProfile. Any theme changes using those
+  /// mechanisms will also trigger this callback. Chrome theme colors will be
+  /// persisted and restored from disk cache with the Chrome runtime, and with
+  /// the Alloy runtime if persist_user_preferences is set to true via
+  /// CefSettings or CefRequestContextSettings.
+  ///
+  /// This callback is not triggered on Window creation so clients that wish to
+  /// customize the initial native/OS theme must call CefWindow::SetThemeColor
+  /// and CefWindow::ThemeChanged before showing the first Window.
+  ///
+  /// Theme colors will be reset to standard values before this callback is
+  /// called for the first affected Window. Call CefWindow::SetThemeColor from
+  /// inside this callback to override a standard color or add a custom color.
+  /// CefViewDelegate::OnThemeChanged will be called after this callback for the
+  /// complete |window| component hierarchy.
+  ///
+  /*--cef()--*/
+  virtual void OnThemeColorsChanged(CefRefPtr<CefWindow> window,
+                                    bool chrome_theme) {}
 };
 
 #endif  // CEF_INCLUDE_VIEWS_CEF_WINDOW_DELEGATE_H_