Reland of Improves focus/activation for aura-mus and DesktopNativeWidgetAura

ui/aura/env_observer.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_AURA_ENV_OBSERVER_H_
 #define UI_AURA_ENV_OBSERVER_H_
 
 #include "ui/aura/aura_export.h"
 
 namespace aura {
 
 class Window;
 class WindowTreeHost;
 
+namespace client {
+class FocusClient;
+}
+
 class AURA_EXPORT EnvObserver {
  public:
   // Called when |window| has been initialized.
   virtual void OnWindowInitialized(Window* window) = 0;
 
  // Called when a WindowTreeHost is initialized.
   virtual void OnHostInitialized(WindowTreeHost* host) {}
 
   // Called when a WindowTreeHost is activated.
   virtual void OnHostActivated(WindowTreeHost* host) {}
 
   // Called right before Env is destroyed.
   virtual void OnWillDestroyEnv() {}
 
+  // Called from Env::SetActiveFocusClient(), see it for details.
+  virtual void OnActiveFocusClientChanged(client::FocusClient* focus_client,
+                                          Window* window) {}
+
  protected:
   virtual ~EnvObserver() {}
 };
 
 }  // namespace aura
 
 #endif  // UI_AURA_ENV_OBSERVER_H_