Adds CaptureClientObserver as a way to track changes in capture

ui/aura/client/capture_client_observer.h

+// Copyright 2016 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_OBSERVER_H_
+#define UI_AURA_CLIENT_CAPTURE_CLIENT_OBSERVER_H_
+
+#include "ui/aura/aura_export.h"
+
+namespace aura {
+class Window;
+
+namespace client {
+
+// Used to observe changes in capture.
+class AURA_EXPORT CaptureClientObserver {
+ public:
+  virtual void OnCaptureChanged(Window* lost_capture,
+                                Window* gained_capture) = 0;
+
+ protected:
+  virtual ~CaptureClientObserver() {}
+};
+
+}  // namespace client
+}  // namespace aura
+
+#endif  // UI_AURA_CLIENT_CAPTURE_CLIENT_OBSERVER_H_