Make CompositorObserver’s destructor protected-and-virtual.

ui/compositor/test/draw_waiter_for_test.h

Index: ui/compositor/test/draw_waiter_for_test.h
diff --git a/ui/compositor/test/draw_waiter_for_test.h b/ui/compositor/test/draw_waiter_for_test.h
index 666d14211039ab96539cb06b5ca342d51c8fe084..f60657cf18d8e9221cd0babc920744ac089c5cdd 100644
--- a/ui/compositor/test/draw_waiter_for_test.h
+++ b/ui/compositor/test/draw_waiter_for_test.h
@@ -38,7 +38,7 @@ class DrawWaiterForTest : public CompositorObserver {
     WAIT_FOR_COMPOSITING_ENDED,
   };
   DrawWaiterForTest(WaitEvent wait_event);
-  ~DrawWaiterForTest();
+  ~DrawWaiterForTest() override;
 
   void WaitImpl(Compositor* compositor);