Explicitly specify the ui::Compositor to DelegatedFrameHost

ui/aura/window_tree_host_observer.h

Index: ui/aura/window_tree_host_observer.h
diff --git a/ui/aura/window_tree_host_observer.h b/ui/aura/window_tree_host_observer.h
index d96e9d3b24dc1230deb2ab5920787bcbff2db98f..a42b5be59a62c0cba958c0929abb6e21c033d69b 100644
--- a/ui/aura/window_tree_host_observer.h
+++ b/ui/aura/window_tree_host_observer.h
@@ -27,6 +27,9 @@ class AURA_EXPORT WindowTreeHostObserver {
   // Called when the native window system sends the host request to close.
   virtual void OnHostCloseRequested(const WindowTreeHost* host) {}
 
+  // Called just before the ui::Compositor for the window tree is destroyed.
+  virtual void OnHostCompositorWillBeDestroyed(const WindowTreeHost* host) {}
+
  protected:
   virtual ~WindowTreeHostObserver() {}
 };