ash/mash: Add a mus-client that sets up ash to provide the system ui for mash.

ash/host/ash_window_tree_host.h

 // Copyright 2014 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 ASH_HOST_ASH_WINDOW_TREE_HOST_H_
 #define ASH_HOST_ASH_WINDOW_TREE_HOST_H_
 
 #include "ash/ash_export.h"
+#include "base/callback_forward.h"
 #include "base/memory/scoped_ptr.h"
 
 namespace aura {
 class Window;
 class WindowTreeHost;
 }
 
 namespace gfx {
 class Insets;
 class Rect;
 }
 
 namespace ui {
 class EventSource;
 class KeyEvent;
 class LocatedEvent;
 }
 
 namespace ash {
 struct AshWindowTreeHostInitParams;
 class InputMethodEventHandler;
 class RootWindowTransformer;
 
 class ASH_EXPORT AshWindowTreeHost {
  public:
+  using Factory =
+      base::Callback<AshWindowTreeHost*(const AshWindowTreeHostInitParams&)>;
   AshWindowTreeHost();
   virtual ~AshWindowTreeHost() {}
 
   // Creates a new AshWindowTreeHost. The caller owns the returned value.
   static AshWindowTreeHost* Create(
       const AshWindowTreeHostInitParams& init_params);
+  static void SetFactory(const Factory& factory);
 
   void set_input_method_handler(InputMethodEventHandler* input_method_handler) {
     input_method_handler_ = input_method_handler;
   }
 
   InputMethodEventHandler* input_method_handler() {
     return input_method_handler_;
   }
 
   // Toggles the host's full screen state.
@@ skipping 23 matching lines @@
   // Translates the native mouse location into screen coordinates.
   void TranslateLocatedEvent(ui::LocatedEvent* event);
 
  private:
   InputMethodEventHandler* input_method_handler_;
 };
 
 }  // namespace ash
 
 #endif  // ASH_HOST_ASH_WINDOW_TREE_HOST_H_