mus: aura::Env should be destroyed last in the mus client.

ui/aura/mus/focus_synchronizer.cc

Index: ui/aura/mus/focus_synchronizer.cc
diff --git a/ui/aura/mus/focus_synchronizer.cc b/ui/aura/mus/focus_synchronizer.cc
index 74ad3e09c8db15283e45a3f2a3818bd6027788a6..341329f3e1e9ef257fd346e5630f8d64834d1e04 100644
--- a/ui/aura/mus/focus_synchronizer.cc
+++ b/ui/aura/mus/focus_synchronizer.cc
@@ -84,13 +84,6 @@ void FocusSynchronizer::OnWindowFocused(Window* gained_focus,
 
 void FocusSynchronizer::OnWindowInitialized(Window* window) {}
 
-void FocusSynchronizer::OnWillDestroyEnv() {
-  // TODO(sky): this override should not be necessary. http://crbug.com/674803.
-  SetActiveFocusClient(nullptr);
-  env_->RemoveObserver(this);
-  env_ = nullptr;
-}
-
 void FocusSynchronizer::OnActiveFocusClientChanged(
     client::FocusClient* focus_client,
     Window* window) {