Adds CaptureClientObserver as a way to track changes in capture

ui/aura/client/default_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_DEFAULT_CAPTURE_CLIENT_H_
 #define UI_AURA_CLIENT_DEFAULT_CAPTURE_CLIENT_H_
 
-#include "base/compiler_specific.h"
 #include "base/macros.h"
+#include "base/observer_list.h"
 #include "ui/aura/aura_export.h"
 #include "ui/aura/client/capture_client.h"
 
 namespace aura {
 namespace client {
 
 class AURA_EXPORT DefaultCaptureClient : public client::CaptureClient {
  public:
-  explicit DefaultCaptureClient(Window* root_window);
+  explicit DefaultCaptureClient(Window* root_window = nullptr);
   ~DefaultCaptureClient() override;
 
  protected:
   // Overridden from client::CaptureClient:
   void SetCapture(Window* window) override;
   void ReleaseCapture(Window* window) override;
   Window* GetCaptureWindow() override;
   Window* GetGlobalCaptureWindow() override;
+  void AddObserver(CaptureClientObserver* observer) override;
+  void RemoveObserver(CaptureClientObserver* observer) override;
 
  private:
-  Window* root_window_;
+  Window* root_window_;  // May be null.
   Window* capture_window_;
+  base::ObserverList<CaptureClientObserver> observers_;
 
   DISALLOW_COPY_AND_ASSIGN(DefaultCaptureClient);
 };
 
 }  // namespace client
 }  // namespace aura
 
 #endif  // UI_AURA_CLIENT_DEFAULT_CAPTURE_CLIENT_H_