Revert of 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 f4285a977d6898236a3c71ce81c6a67101898392..c4f56791d4505e200493b03b7155952276902ca3 100644
--- a/ui/views/view_observer.h
+++ b/ui/views/view_observer.h
@@ -35,9 +35,6 @@
   // 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) {}