exo: Make sure vsync observer is removed when Surface is destroyed.

If the window is still in the tree when Surface is destroyed
then we need to remove the vsync observer in the dtor.

BUG=679787

Review-Url: https://codereview.chromium.org/2627713004
Cr-Commit-Position: refs/heads/master@{#442996}

components/exo/surface.cc

@@ -205,6 +205,8 @@ Surface::~Surface() {
     observer.OnSurfaceDestroying(this);
 
   window_->RemoveObserver(this);
+  if (window_->layer()->GetCompositor())
+    window_->layer()->GetCompositor()->vsync_manager()->RemoveObserver(this);
   window_->layer()->SetShowSolidColorContent();
 
   frame_callbacks_.splice(frame_callbacks_.end(), pending_frame_callbacks_);