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

ui/views/mus/window_tree_host_mus.h

Index: ui/views/mus/window_tree_host_mus.h
diff --git a/ui/views/mus/window_tree_host_mus.h b/ui/views/mus/window_tree_host_mus.h
index ec41685c9bb8e948c540e53a9dce268f6abd6e68..a97475aba0cae91490b9bd406ee2d4be27437d8e 100644
--- a/ui/views/mus/window_tree_host_mus.h
+++ b/ui/views/mus/window_tree_host_mus.h
@@ -12,12 +12,8 @@
 class SkBitmap;
 
 namespace mojo {
-namespace shell {
-namespace mojom {
 class Shell;
 }
-}
-}
 
 namespace mus {
 class Window;
@@ -31,7 +27,7 @@ class PlatformWindowMus;
 
 class VIEWS_MUS_EXPORT WindowTreeHostMus : public aura::WindowTreeHostPlatform {
  public:
-  WindowTreeHostMus(mojo::shell::mojom::Shell* shell,
+  WindowTreeHostMus(mojo::Shell* shell,
                     NativeWidgetMus* native_widget_,
                     mus::Window* window);
   ~WindowTreeHostMus() override;