Explicitly specify the ui::Compositor to DelegatedFrameHost

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 76b6c5b431d4cccd9b249e2e71ef9745ff8a9d0a..09009435a60c1a68790e8c6967100e447fc88097 100644
--- a/ui/compositor/compositor_observer.h
+++ b/ui/compositor/compositor_observer.h
@@ -41,12 +41,10 @@ class COMPOSITOR_EXPORT CompositorObserver {
   virtual void OnCompositingLockStateChanged(Compositor* compositor) = 0;
 
  protected:
-#if defined(OS_MACOSX)
-  // Debugging instrumentation for crbug.com/401630.
-  // TODO(ccameron): remove this.
+  // Instrumentation to ensure that observers are not left attached after they
+  // are destroyed.
   friend class Compositor;
   int32 observing_count_;
-#endif
 
   virtual ~CompositorObserver();
 };