mus: Fix a teardown crash.

services/ui/ws/frame_generator.cc

Index: services/ui/ws/frame_generator.cc
diff --git a/services/ui/ws/frame_generator.cc b/services/ui/ws/frame_generator.cc
index d42f96d1143200fcc394bd94eccf228ecbb672ba..f02a08a147218d83ecc3bce686b7bc56c043eb95 100644
--- a/services/ui/ws/frame_generator.cc
+++ b/services/ui/ws/frame_generator.cc
@@ -254,8 +254,7 @@ void FrameGenerator::AddOrUpdateSurfaceReference(
     dependencies_[surface_id.frame_sink_id()] = dependency;
     // Observe |window_surface|'s window so that we can release references when
     // the window is destroyed.
-    if (!window_surface->window()->HasObserver(this))
-      window_surface->window()->AddObserver(this);
+    Add(window_surface->window());
     return;
   }
 
@@ -300,7 +299,7 @@ void FrameGenerator::ReleaseAllSurfaceReferences() {
 }
 
 void FrameGenerator::OnWindowDestroying(ServerWindow* window) {
-  window->RemoveObserver(this);
+  ServerWindowTracker::OnWindowDestroying(window);

[sky, 2016/10/26 17:48:41]

Seems weird to call OnWindowDestroying (which is really an implementation
detail). Can you call Remove() here?

[sadrul, 2016/10/27 04:59:44]

Done.

   ServerWindowCompositorFrameSinkManager* surface_manager =
       window->compositor_frame_sink_manager();
   // If FrameGenerator was observing |window|, then that means it had a surface