Make CompositorObserver’s destructor protected-and-virtual.

ui/compositor/compositor_observer.h

 // 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.
 
 #ifndef UI_COMPOSITOR_COMPOSITOR_OBSERVER_H_
 #define UI_COMPOSITOR_COMPOSITOR_OBSERVER_H_
 
 #include "base/time/time.h"
 #include "ui/compositor/compositor_export.h"
 
@@ skipping 17 matching lines @@
   // account and has issued the graphics commands.
   virtual void OnCompositingStarted(Compositor* compositor,
                                     base::TimeTicks start_time) = 0;
 
   // Called when the compositor lock state changes.
   virtual void OnCompositingLockStateChanged(Compositor* compositor) = 0;
 
   // Called at the top of the compositor's destructor, to give observers a
   // chance to remove themselves.
   virtual void OnCompositingShuttingDown(Compositor* compositor) = 0;
+
+  virtual ~CompositorObserver() {}
 };
 
 }  // namespace ui
 
 #endif  // UI_COMPOSITOR_COMPOSITOR_OBSERVER_H_