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

ui/views/view_observer.h

 // Copyright 2016 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.
 
 #ifndef UI_VIEWS_VIEW_OBSERVER_H_
 #define UI_VIEWS_VIEW_OBSERVER_H_
 
 #include "ui/views/views_export.h"
 
 namespace views {
@@ skipping 17 matching lines @@
   // Called when View::SetEnabled() is called with a new value.
   virtual void OnViewEnabledChanged(View* observed_view) {}
 
   // Called when the bounds of |observed_view| change.
   virtual void OnViewBoundsChanged(View* observed_view) {}
 
   // Called when a child is reordered among its siblings, specifically
   // View::ReorderChildView() is called on |observed_view|.
   virtual void OnChildViewReordered(View* observed_view, View* child) {}
 
+  // FIXME

[msw, 2017/04/14 20:42:17]

FIXME

[Evan Stade, 2017/04/17 15:20:03]

Done.

+  virtual void OnViewNativeThemeChanged(View* observed_view) {}
+
   // Called from ~View.
   virtual void OnViewIsDeleting(View* observed_view) {}
 
  protected:
   virtual ~ViewObserver() {}
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_VIEW_OBSERVER_H_