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.

patch/patches/chrome_runtime_views.patch

@@ -233,7 +233,7 @@ index 0ccfe39eb5696..c9424316b6d14 100644
      return gfx::Rect();
    }
 diff --git chrome/browser/ui/views/frame/browser_frame.cc chrome/browser/ui/views/frame/browser_frame.cc
-index cce1835d35be3..5ac27ca62876a 100644
+index cce1835d35be3..75f8aae063ada 100644
 --- chrome/browser/ui/views/frame/browser_frame.cc
 +++ chrome/browser/ui/views/frame/browser_frame.cc
 @@ -114,15 +114,23 @@ ui::ColorProviderKey::SchemeVariant GetSchemeVariant(
@@ -341,7 +341,21 @@ index cce1835d35be3..5ac27ca62876a 100644
  
    key.app_controller = browser_view_->browser()->app_controller();
  
-@@ -637,5 +671,8 @@ bool BrowserFrame::RegenerateFrameOnThemeChange(
+@@ -572,6 +606,13 @@ void BrowserFrame::SelectNativeTheme() {
+     return;
+   }
+ 
++  // Always use the NativeTheme for forced color modes.
++  if (ui::NativeTheme::IsForcedDarkMode() ||
++      ui::NativeTheme::IsForcedLightMode()) {
++    SetNativeTheme(native_theme);
++    return;
++  }
++
+   // Ignore the system theme for web apps with window-controls-overlay as the
+   // display_override so the web contents can blend with the overlay by using
+   // the developer-provided theme color for a better experience. Context:
+@@ -637,5 +678,8 @@ bool BrowserFrame::RegenerateFrameOnThemeChange(
  }
  
  bool BrowserFrame::IsIncognitoBrowser() const {
@@ -351,7 +365,7 @@ index cce1835d35be3..5ac27ca62876a 100644
    return browser_view_->browser()->profile()->IsIncognitoProfile();
  }
 diff --git chrome/browser/ui/views/frame/browser_frame.h chrome/browser/ui/views/frame/browser_frame.h
-index 2e973c9e279b0..12b62efb8071f 100644
+index 2e973c9e279b0..8662f9cf14b17 100644
 --- chrome/browser/ui/views/frame/browser_frame.h
 +++ chrome/browser/ui/views/frame/browser_frame.h
 @@ -58,7 +58,9 @@ enum class TabDragKind {
@@ -364,6 +378,38 @@ index 2e973c9e279b0..12b62efb8071f 100644
  
    BrowserFrame(const BrowserFrame&) = delete;
    BrowserFrame& operator=(const BrowserFrame&) = delete;
+@@ -137,7 +139,7 @@ class BrowserFrame : public views::Widget, public views::ContextMenuController {
+ 
+   // ThemeService calls this when a user has changed their theme, indicating
+   // that it's time to redraw everything.
+-  void UserChangedTheme(BrowserThemeChangeType theme_change_type);
++  virtual void UserChangedTheme(BrowserThemeChangeType theme_change_type);
+ 
+   // views::Widget:
+   views::internal::RootView* CreateRootView() override;
+@@ -175,17 +177,17 @@ class BrowserFrame : public views::Widget, public views::ContextMenuController {
+   void OnNativeThemeUpdated(ui::NativeTheme* observed_theme) override;
+   ui::ColorProviderKey GetColorProviderKey() const override;
+ 
++  // Select a native theme that is appropriate for the current context. This is
++  // currently only needed for Linux to switch between the regular NativeTheme
++  // and the GTK NativeTheme instance.
++  void SelectNativeTheme();
++
+  private:
+   void OnTouchUiChanged();
+ 
+   // Callback for MenuRunner.
+   void OnMenuClosed();
+ 
+-  // Select a native theme that is appropriate for the current context. This is
+-  // currently only needed for Linux to switch between the regular NativeTheme
+-  // and the GTK NativeTheme instance.
+-  void SelectNativeTheme();
+-
+   // Regenerate the frame on theme change if necessary. Returns true if
+   // regenerated.
+   bool RegenerateFrameOnThemeChange(BrowserThemeChangeType theme_change_type);
 diff --git chrome/browser/ui/views/frame/browser_view.cc chrome/browser/ui/views/frame/browser_view.cc
 index f764d5edc704c..b7d5ee188736e 100644
 --- chrome/browser/ui/views/frame/browser_view.cc