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.

libcef/browser/chrome/views/chrome_browser_frame.h

@@ -6,6 +6,9 @@
 #define CEF_LIBCEF_BROWSER_CHROME_VIEWS_CHROME_BROWSER_FRAME_H_
 #pragma once
 
+#include "libcef/browser/views/color_provider_tracker.h"
+#include "libcef/browser/views/widget.h"
+
 #include "chrome/browser/ui/browser.h"
 #include "chrome/browser/ui/views/frame/browser_frame.h"
 
@@ -86,18 +89,34 @@
 // modifications.
 
 class BrowserView;
+class CefWindowView;
 
 // Widget for a Views-hosted Chrome browser. Created in
 // CefWindowView::CreateWidget() when the Chrome runtime is enabled.
-class ChromeBrowserFrame : public BrowserFrame {
+class ChromeBrowserFrame : public BrowserFrame,
+                           public CefWidget,
+                           public CefColorProviderTracker::Observer {
  public:
-  ChromeBrowserFrame() = default;
+  explicit ChromeBrowserFrame(CefWindowView* window_view);
+
   ChromeBrowserFrame(const ChromeBrowserFrame&) = delete;
   ChromeBrowserFrame& operator=(const ChromeBrowserFrame&) = delete;
 
+  // Called from ChromeBrowserView::InitBrowser after |browser| creation.
   void Init(BrowserView* browser_view, std::unique_ptr<Browser> browser);
 
-  void ToggleFullscreenMode();
+  // CefWidget methods:
+  views::Widget* GetWidget() override { return this; }
+  const views::Widget* GetWidget() const override { return this; }
+  void Initialized() override;
+  bool IsInitialized() const override { return initialized_; }
+  void AddAssociatedProfile(Profile* profile) override;
+  void RemoveAssociatedProfile(Profile* profile) override;
+  Profile* GetThemeProfile() const override;
+  bool ToggleFullscreenMode() override;
+
+  // BrowserFrame methods:
+  void UserChangedTheme(BrowserThemeChangeType theme_change_type) override;
 
   // views::Widget methods:
   views::internal::RootView* CreateRootView() override;
@@ -105,10 +124,25 @@ class ChromeBrowserFrame : public BrowserFrame {
       override;
   void Activate() override;
 
+  // NativeWidgetDelegate methods:
+  void OnNativeWidgetDestroyed() override;
+
+  // ui::NativeThemeObserver methods:
+  void OnNativeThemeUpdated(ui::NativeTheme* observed_theme) override;
+
   BrowserView* browser_view() const { return browser_view_; }
 
  private:
+  // CefColorProviderTracker::Observer methods:
+  void OnColorProviderCacheResetMissed() override;
+
+  CefWindowView* window_view_;
   BrowserView* browser_view_ = nullptr;
+
+  bool initialized_ = false;
+  bool native_theme_change_ = false;
+
+  CefColorProviderTracker color_provider_tracker_{this};
 };
 
 #endif  // CEF_LIBCEF_BROWSER_CHROME_VIEWS_CHROME_BROWSER_FRAME_H_