mus: Fix handled status in UI event ack, add MessageLoop::NestingObserver

ui/views/mus/native_widget_mus.h

 // Copyright 2015 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_NATIVE_WIDGET_MUS_H_
 #define UI_VIEWS_MUS_NATIVE_WIDGET_MUS_H_
 
 #include <stdint.h>
 
 #include <map>
@@ skipping 55 matching lines @@
   // Configures the set of properties supplied to the window manager when
   // creating a new Window for a Widget.
   static void ConfigurePropertiesForNewWindow(
       const Widget::InitParams& init_params,
       std::map<std::string, std::vector<uint8_t>>* properties);
 
   // Notifies all widgets the frame constants changed in some way.
   static void NotifyFrameChanged(mus::WindowTreeConnection* connection);
 
   mus::Window* window() { return window_; }
+  WindowTreeHostMus* window_tree_host() { return window_tree_host_.get(); }
 
   aura::Window* GetRootWindow();
 
   void OnPlatformWindowClosed();
   void OnActivationChanged(bool active);
 
  protected:
   // Updates the client area in the mus::Window.
   virtual void UpdateClientArea();
 
@@ skipping 142 matching lines @@
   std::unique_ptr<aura::client::WindowTreeClient> window_tree_client_;
   std::unique_ptr<aura::client::ScreenPositionClient> screen_position_client_;
   base::WeakPtrFactory<NativeWidgetMus> close_widget_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(NativeWidgetMus);
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_MUS_NATIVE_WIDGET_MUS_H_