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/request_context_impl.h

@@ -131,6 +131,11 @@ class CefRequestContextImpl : public CefRequestContext {
                          const CefString& top_level_url,
                          cef_content_setting_types_t content_type,
                          cef_content_setting_values_t value) override;
+  void SetChromeColorScheme(cef_color_variant_t variant,
+                            cef_color_t user_color) override;
+  cef_color_variant_t GetChromeColorSchemeMode() override;
+  cef_color_t GetChromeColorSchemeColor() override;
+  cef_color_variant_t GetChromeColorSchemeVariant() override;
 
   const CefRequestContextSettings& settings() const { return config_.settings; }
 
@@ -205,6 +210,10 @@ class CefRequestContextImpl : public CefRequestContext {
       cef_content_setting_types_t content_type,
       cef_content_setting_values_t value,
       CefBrowserContext::Getter browser_context_getter);
+  void SetChromeColorSchemeInternal(
+      cef_color_variant_t variant,
+      cef_color_t user_color,
+      CefBrowserContext::Getter browser_context_getter);
 
   void InitializeCookieManagerInternal(
       CefRefPtr<CefCookieManagerImpl> cookie_manager,