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

mash/wm/root_window_controller.h

Index: mash/wm/root_window_controller.h
diff --git a/mash/wm/root_window_controller.h b/mash/wm/root_window_controller.h
index 87e6158918ca3f52e79032ab329cbf99e55d60d4..094e59e70e1822f21f94b3ba1b0b57b92ab41706 100644
--- a/mash/wm/root_window_controller.h
+++ b/mash/wm/root_window_controller.h
@@ -13,12 +13,8 @@
 #include "mojo/public/cpp/bindings/binding.h"
 
 namespace mojo {
-namespace shell {
-namespace mojom {
 class Shell;
 }
-}
-}
 
 namespace mus {
 class WindowManagerClient;
@@ -52,7 +48,7 @@ class RootWindowController : public mus::WindowObserver,
   // Deletes this.
   void Destroy();
 
-  mojo::shell::mojom::Shell* GetShell();
+  mojo::Shell* GetShell();
 
   mus::Window* root() { return root_; }