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

ui/views/mus/platform_window_mus.h

Index: ui/views/mus/platform_window_mus.h
diff --git a/ui/views/mus/platform_window_mus.h b/ui/views/mus/platform_window_mus.h
index d4d19cb7c85146b2e0a551bc5112fd0fdf91936e..ee124f5f6b2c4a9828a60cb0b1098a0bbf0b561c 100644
--- a/ui/views/mus/platform_window_mus.h
+++ b/ui/views/mus/platform_window_mus.h
@@ -21,12 +21,8 @@ class BitmapUploader;
 }
 
 namespace mojo {
-namespace shell {
-namespace mojom {
 class Shell;
 }
-}
-}
 
 namespace ui {
 class ViewProp;
@@ -40,7 +36,7 @@ class VIEWS_MUS_EXPORT PlatformWindowMus
       public NON_EXPORTED_BASE(mus::InputEventHandler) {
  public:
   PlatformWindowMus(ui::PlatformWindowDelegate* delegate,
-                    mojo::shell::mojom::Shell* shell,
+                    mojo::Shell* shell,
                     mus::Window* mus_window);
   ~PlatformWindowMus() override;