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

ash/host/ash_window_tree_host_platform.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 ASH_HOST_ASH_WINDOW_TREE_HOST_PLATFORM_H_
 #define ASH_HOST_ASH_WINDOW_TREE_HOST_PLATFORM_H_
 
+#include "ash/ash_export.h"
 #include "ash/host/ash_window_tree_host.h"
 #include "ash/host/transformer_helper.h"
 #include "ui/aura/window_tree_host_platform.h"
 
 namespace ash {
 
-class AshWindowTreeHostPlatform : public AshWindowTreeHost,
-                                  public aura::WindowTreeHostPlatform {
+class ASH_EXPORT AshWindowTreeHostPlatform
+    : public AshWindowTreeHost,
+      public aura::WindowTreeHostPlatform {
  public:
   explicit AshWindowTreeHostPlatform(const gfx::Rect& initial_bounds);
   ~AshWindowTreeHostPlatform() override;
 
  protected:
   AshWindowTreeHostPlatform();
 
   // AshWindowTreeHost:
   void ToggleFullScreen() override;
   bool ConfineCursorToRootWindow() override;
@@ skipping 20 matching lines @@
   void SetTapToClickPaused(bool state);
 
   TransformerHelper transformer_helper_;
 
   DISALLOW_COPY_AND_ASSIGN(AshWindowTreeHostPlatform);
 };
 
 }  // namespace ash
 
 #endif  // ASH_HOST_ASH_WINDOW_TREE_HOST_PLATFORM_H_