Implement hit tests/client area.

ui/views/mus/mus_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_VIEWS_MUS_MUS_CLIENT_OBSERVER_H_
+#define UI_VIEWS_MUS_MUS_CLIENT_OBSERVER_H_
+
+#include "ui/views/mus/mus_export.h"
+
+namespace views {
+
+// Allows individual DesktopWindowTreeHostsMus to observe MusClient events.
+class VIEWS_MUS_EXPORT MusClientObserver {
+ public:
+  // Notification that windows should update per window values from the
+  // WindowManagerFrameValues.
+  virtual void OnWindowManagerFrameValuesChanged() = 0;
+
+ protected:
+  virtual ~MusClientObserver() {}
+};
+
+}  // namespace views
+
+#endif  // UI_VIEWS_MUS_MUS_CLIENT_OBSERVER_H_