Change FocusSynchronizer to maintain active focus client and window.

ui/aura/mus/focus_synchronizer_observer.h

+// Copyright 2017 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_MUS_FOCUS_SYNCHRONIZER_OBSERVER_H_
+#define UI_AURA_MUS_FOCUS_SYNCHRONIZER_OBSERVER_H_
+
+#include "ui/aura/aura_export.h"
+
+namespace aura {
+class Window;
+
+namespace client {
+class FocusClient;
+}
+
+class AURA_EXPORT FocusSynchronizerObserver {
+ public:
+  // Called from FocusSynchronizer::OnActiveFocusClientChanged() to notify its
+  // observers of active focus client and focus window changes.
+  virtual void OnActiveFocusClientChanged(client::FocusClient* focus_client,
+                                          Window* window) {}
+
+ protected:
+  virtual ~FocusSynchronizerObserver() {}
+};
+
+}  // namespace aura
+
+#endif  // UI_AURA_MUS_FOCUS_SYNCHRONIZER_OBSERVER_H_