Extract shell methods from ApplicationImpl into a base class, and pass this to Initialize() instead.

components/mus/ws/window_tree_host_impl.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 COMPONENTS_MUS_WS_WINDOW_TREE_HOST_IMPL_H_
 #define COMPONENTS_MUS_WS_WINDOW_TREE_HOST_IMPL_H_
 
 #include <stdint.h>
 
 #include <queue>
@@ skipping 30 matching lines @@
                            public mojom::WindowTreeHost,
                            public FocusControllerObserver,
                            public FocusControllerDelegate,
                            public EventDispatcherDelegate,
                            public ServerWindowObserver {
  public:
   // TODO(fsamuel): All these parameters are just plumbing for creating
   // DisplayManagers. We should probably just store these common parameters
   // in the DisplayManagerFactory and pass them along on DisplayManager::Create.
   WindowTreeHostImpl(ConnectionManager* connection_manager,
-                     mojo::ApplicationImpl* app_impl,
+                     mojo::Shell* shell,
                      const scoped_refptr<GpuState>& gpu_state,
                      const scoped_refptr<SurfacesState>& surfaces_state);
   ~WindowTreeHostImpl() override;
 
   // Initializes state that depends on the existence of a WindowTreeHostImpl.
   void Init(WindowTreeHostDelegate* delegate);
 
   uint32_t id() const { return id_; }
 
   const WindowTreeImpl* GetWindowTree() const;
@@ skipping 156 matching lines @@
 
   mojom::FrameDecorationValuesPtr frame_decoration_values_;
 
   DISALLOW_COPY_AND_ASSIGN(WindowTreeHostImpl);
 };
 
 }  // namespace ws
 }  // namespace mus
 
 #endif  // COMPONENTS_MUS_WS_WINDOW_TREE_HOST_IMPL_H_