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

ui/views/mus/native_widget_mus.h

Index: ui/views/mus/native_widget_mus.h
diff --git a/ui/views/mus/native_widget_mus.h b/ui/views/mus/native_widget_mus.h
index c98bed0640fa75cd5707ac311e7c6585d42c98aa..caf889cc32059be756ff0da137ecf387ef388aec 100644
--- a/ui/views/mus/native_widget_mus.h
+++ b/ui/views/mus/native_widget_mus.h
@@ -29,12 +29,8 @@ class Window;
 }
 
 namespace mojo {
-namespace shell {
-namespace mojom {
 class Shell;
 }
-}
-}
 
 namespace mus {
 class Window;
@@ -62,7 +58,7 @@ class VIEWS_MUS_EXPORT NativeWidgetMus : public internal::NativeWidgetPrivate,
                                          public aura::WindowTreeHostObserver {
  public:
   NativeWidgetMus(internal::NativeWidgetDelegate* delegate,
-                  mojo::shell::mojom::Shell* shell,
+                  mojo::Shell* shell,
                   mus::Window* window,
                   mus::mojom::SurfaceType surface_type);
   ~NativeWidgetMus() override;
@@ -204,7 +200,7 @@ class VIEWS_MUS_EXPORT NativeWidgetMus : public internal::NativeWidgetPrivate,
  private:
   mus::Window* window_;
 
-  mojo::shell::mojom::Shell* shell_;
+  mojo::Shell* shell_;
 
   internal::NativeWidgetDelegate* native_widget_delegate_;