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 d5f3371f5a734a3b1bf4eaf2b087fe46a46c5035..5c0d3db21f90d9d09f07c1c19887edf429dbc8b8 100644
--- a/ui/compositor/compositor_observer.h
+++ b/ui/compositor/compositor_observer.h
@@ -15,6 +15,8 @@ class Compositor;
 // A compositor observer is notified when compositing completes.
 class COMPOSITOR_EXPORT CompositorObserver {
  public:
+  virtual ~CompositorObserver() = default;
+
   // A commit proxies information from the main thread to the compositor
   // thread. It typically happens when some state changes that will require a
   // composite. In the multi-threaded case, many commits may happen between