Introduce a type of View background that stays in sync with its host

ui/views/view.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #define _USE_MATH_DEFINES // For VC++ to get M_PI. This has to be first.
 
 #include "ui/views/view.h"
 
 #include <algorithm>
 #include <cmath>
@@ skipping 2100 matching lines @@
 void View::PropagateNativeThemeChanged(const ui::NativeTheme* theme) {
   if (native_theme_ && native_theme_ != theme)
     return;
 
   {
     internal::ScopedChildrenLock lock(this);
     for (auto* child : children_)
       child->PropagateNativeThemeChanged(theme);
   }
   OnNativeThemeChanged(theme);
+  for (ViewObserver& observer : observers_)
+    observer.OnViewNativeThemeChanged(this);
 }
 
 // Size and disposition --------------------------------------------------------
 
 void View::PropagateVisibilityNotifications(View* start, bool is_visible) {
   {
     internal::ScopedChildrenLock lock(this);
     for (auto* child : children_)
       child->PropagateVisibilityNotifications(start, is_visible);
   }
@@ skipping 513 matching lines @@
   // Message the RootView to do the drag and drop. That way if we're removed
   // the RootView can detect it and avoid calling us back.
   gfx::Point widget_location(event.location());
   ConvertPointToWidget(this, &widget_location);
   widget->RunShellDrag(this, data, widget_location, drag_operations, source);
   // WARNING: we may have been deleted.
   return true;
 }
 
 }  // namespace views