Make CompositorObserver’s destructor protected-and-virtual.

ui/compositor/compositor_observer.h

Index: ui/compositor/compositor_observer.h
diff --git a/ui/compositor/compositor_observer.h b/ui/compositor/compositor_observer.h
index dcdc58384f57a2fe7b697631cf28312cec36205e..90e859a7c7f31f38af0ed5e104482d92c812d53c 100644
--- a/ui/compositor/compositor_observer.h
+++ b/ui/compositor/compositor_observer.h
@@ -35,6 +35,8 @@ class COMPOSITOR_EXPORT CompositorObserver {
   // 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