Adds CaptureClientObserver as a way to track changes in capture

ui/views/widget/desktop_aura/desktop_capture_client.h

 // Copyright 2013 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_VIEWS_WIDGET_DESKTOP_AURA_DESKTOP_CAPTURE_CLIENT_H_
 #define UI_VIEWS_WIDGET_DESKTOP_AURA_DESKTOP_CAPTURE_CLIENT_H_
 
 #include <set>
 
 #include "base/compiler_specific.h"
@@ skipping 24 matching lines @@
 class VIEWS_EXPORT DesktopCaptureClient : public aura::client::CaptureClient {
  public:
   explicit DesktopCaptureClient(aura::Window* root);
   ~DesktopCaptureClient() override;
 
   // Overridden from aura::client::CaptureClient:
   void SetCapture(aura::Window* window) override;
   void ReleaseCapture(aura::Window* window) override;
   aura::Window* GetCaptureWindow() override;
   aura::Window* GetGlobalCaptureWindow() override;
+  void AddObserver(aura::client::CaptureClientObserver* observer) override;
+  void RemoveObserver(aura::client::CaptureClientObserver* observer) override;
 
  private:
   typedef std::set<DesktopCaptureClient*> CaptureClients;
 
   aura::Window* root_;
   aura::Window* capture_window_;
 
   // Set of DesktopCaptureClients.
   static CaptureClients* capture_clients_;
 
   DISALLOW_COPY_AND_ASSIGN(DesktopCaptureClient);
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_WIDGET_DESKTOP_AURA_DESKTOP_CAPTURE_CLIENT_H_