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

ui/views/view_observer.h

Index: ui/views/view_observer.h
diff --git a/ui/views/view_observer.h b/ui/views/view_observer.h
index c4f56791d4505e200493b03b7155952276902ca3..f4285a977d6898236a3c71ce81c6a67101898392 100644
--- a/ui/views/view_observer.h
+++ b/ui/views/view_observer.h
@@ -35,6 +35,9 @@ class VIEWS_EXPORT ViewObserver {
   // View::ReorderChildView() is called on |observed_view|.
   virtual void OnChildViewReordered(View* observed_view, View* child) {}
 
+  // Called when the active NativeTheme has changed for |observed_view|.
+  virtual void OnViewNativeThemeChanged(View* observed_view) {}
+
   // Called from ~View.
   virtual void OnViewIsDeleting(View* observed_view) {}