Creates BackgroundShell and uses it in test

ui/views/mus/window_manager_connection.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_WINDOW_MANAGER_CONNECTION_H_
 #define UI_VIEWS_MUS_WINDOW_MANAGER_CONNECTION_H_
 
 #include <stdint.h>
 
 #include "base/macros.h"
@@ skipping 22 matching lines @@
 // WindowManagerConnection is a singleton and should be created early on.
 //
 // TODO(sky): this name is now totally confusing. Come up with a better one.
 class VIEWS_MUS_EXPORT WindowManagerConnection
     : public NON_EXPORTED_BASE(mus::WindowTreeDelegate),
       public ScreenMusDelegate {
  public:
   static void Create(mojo::Shell* shell);
   static WindowManagerConnection* Get();
 
+  // Destroys the singleton instance.
+  static void Reset();
+
   mojo::Shell* shell() { return shell_; }
 
   mus::Window* NewWindow(const std::map<std::string,
                          std::vector<uint8_t>>& properties);
 
  private:
   explicit WindowManagerConnection(mojo::Shell* shell);
   ~WindowManagerConnection() override;
 
   // mus::WindowTreeDelegate:
   void OnEmbed(mus::Window* root) override;
   void OnConnectionLost(mus::WindowTreeConnection* connection) override;
 
   // ScreenMusDelegate:
   void OnWindowManagerFrameValuesChanged() override;
 
   NativeWidget* CreateNativeWidget(const Widget::InitParams& init_params,
                                    internal::NativeWidgetDelegate* delegate);
 
   mojo::Shell* shell_;
   scoped_ptr<ScreenMus> screen_;
   scoped_ptr<mus::WindowTreeConnection> window_tree_connection_;
 
   DISALLOW_COPY_AND_ASSIGN(WindowManagerConnection);
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_MUS_WINDOW_MANAGER_CONNECTION_H_