Revert of Remove CompositorObserver::OnCompositingEnded()

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 0fbb032c1946f26b15bdec6fbae745f85520532f..5c0d3db21f90d9d09f07c1c19887edf429dbc8b8 100644
--- a/ui/compositor/compositor_observer.h
+++ b/ui/compositor/compositor_observer.h
@@ -31,6 +31,9 @@ class COMPOSITOR_EXPORT CompositorObserver {
   virtual void OnCompositingStarted(Compositor* compositor,
                                     base::TimeTicks start_time) = 0;
 
+  // Called when compositing completes: the present to screen has completed.
+  virtual void OnCompositingEnded(Compositor* compositor) = 0;
+
   // Called when the compositor lock state changes.
   virtual void OnCompositingLockStateChanged(Compositor* compositor) = 0;