Adds CaptureClientObserver as a way to track changes in capture

ui/aura/client/capture_client.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_CLIENT_CAPTURE_CLIENT_H_
 #define UI_AURA_CLIENT_CAPTURE_CLIENT_H_
 
 #include "ui/aura/aura_export.h"
 
 namespace aura {
 class Window;
 
 namespace client {
 
+class CaptureClientObserver;
+
 // An interface implemented by an object that manages input capture.
 class AURA_EXPORT CaptureClient {
  public:
   // Does a capture on the |window|.
   virtual void SetCapture(Window* window) = 0;
 
   // Releases a capture from the |window|.
   virtual void ReleaseCapture(Window* window) = 0;
 
   // Returns the current capture window. This may only return a Window if the
   // Window that has capture is a child of the Window the CaptureClient is
   // installed on. GetGlobalCaptureWindow() can be used to locate the Window
   // that has capture regardless of the Window the CaptureClient is installed
   // on.
   virtual Window* GetCaptureWindow() = 0;
 
   // See description of GetCaptureWindow() for details.
   virtual Window* GetGlobalCaptureWindow() = 0;
 
+  virtual void AddObserver(CaptureClientObserver* observer) = 0;
+  virtual void RemoveObserver(CaptureClientObserver* observer) = 0;
+
  protected:
   virtual ~CaptureClient() {}
 };
 
 // Sets/Gets the capture client on the root Window.
 AURA_EXPORT void SetCaptureClient(Window* root_window,
                                   CaptureClient* client);
 AURA_EXPORT CaptureClient* GetCaptureClient(Window* root_window);
 
 // A utility function to get the current capture window. Returns NULL
 // if the window doesn't have a root window, or there is no capture window.
 AURA_EXPORT Window* GetCaptureWindow(Window* window);
 
 }  // namespace client
 }  // namespace aura
 
 #endif  // UI_AURA_CLIENT_CAPTURE_CLIENT_H_