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/views/button_impl.h

@@ -79,13 +79,14 @@ CEF_BUTTON_IMPL_T cef_button_state_t CEF_BUTTON_IMPL_D::GetState() {
 
 CEF_BUTTON_IMPL_T void CEF_BUTTON_IMPL_D::SetInkDropEnabled(bool enabled) {
   CEF_REQUIRE_VALID_RETURN_VOID();
-  views::InkDrop::Get(ParentClass::root_view())
-      ->SetMode(enabled ? views::InkDropHost::InkDropMode::ON
-                        : views::InkDropHost::InkDropMode::OFF);
+  auto* inkdrop = views::InkDrop::Get(ParentClass::root_view());
+  inkdrop->SetMode(enabled ? views::InkDropHost::InkDropMode::ON
+                           : views::InkDropHost::InkDropMode::OFF);
   if (enabled) {
-    views::InkDrop::Get(ParentClass::root_view())
-        ->SetBaseColor(color_utils::BlendTowardMaxContrast(
-            ParentClass::GetBackgroundColor(), 0x61));
+    // Never returns an empty value.
+    const auto& color = view_util::GetBackgroundColor(
+        ParentClass::root_view(), /*allow_transparent=*/false);
+    inkdrop->SetBaseColor(color_utils::BlendTowardMaxContrast(*color, 0x61));
   }
 }