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

ui/views/mus/native_widget_mus.cc

 // 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.
 
 #include "ui/views/mus/native_widget_mus.h"
 
 #include "base/macros.h"
 #include "base/thread_task_runner_handle.h"
 #include "components/mus/public/cpp/property_type_converters.h"
 #include "components/mus/public/cpp/window.h"
@@ skipping 157 matching lines @@
     behavior |= mus::mojom::kResizeBehaviorCanMinimize;
   return behavior;
 }
 
 }  // namespace
 
 ////////////////////////////////////////////////////////////////////////////////
 // NativeWidgetMus, public:
 
 NativeWidgetMus::NativeWidgetMus(internal::NativeWidgetDelegate* delegate,
-                                 mojo::shell::mojom::Shell* shell,
+                                 mojo::Shell* shell,
                                  mus::Window* window,
                                  mus::mojom::SurfaceType surface_type)
     : window_(window),
       shell_(shell),
       native_widget_delegate_(delegate),
       surface_type_(surface_type),
       show_state_before_fullscreen_(ui::PLATFORM_WINDOW_STATE_UNKNOWN),
       ownership_(Widget::InitParams::NATIVE_WIDGET_OWNS_WIDGET),
       content_(new aura::Window(this)),
       close_widget_factory_(this) {
@@ skipping 659 matching lines @@
 
 void NativeWidgetMus::OnGestureEvent(ui::GestureEvent* event) {
   native_widget_delegate_->OnGestureEvent(event);
 }
 
 void NativeWidgetMus::OnHostCloseRequested(const aura::WindowTreeHost* host) {
   GetWidget()->Close();
 }
 
 }  // namespace views