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

components/mus/public/cpp/window_tree_connection.h

Index: components/mus/public/cpp/window_tree_connection.h
diff --git a/components/mus/public/cpp/window_tree_connection.h b/components/mus/public/cpp/window_tree_connection.h
index bd1519b70eea90e386285b9cf774f7f9b62daa32..da5b06f7238ceb430261bca8ad05a87d8b8a3862 100644
--- a/components/mus/public/cpp/window_tree_connection.h
+++ b/components/mus/public/cpp/window_tree_connection.h
@@ -16,7 +16,7 @@
 #include "mojo/public/cpp/bindings/interface_request.h"
 
 namespace mojo {
-class ApplicationImpl;
+class Shell;
 }
 
 namespace mus {
@@ -43,7 +43,7 @@ class WindowTreeConnection {
   // Creates a WindowTreeConnection with no roots. Use this to establish a
   // connection directly to mus and create top level windows.
   static WindowTreeConnection* Create(WindowTreeDelegate* delegate,
-                                      mojo::ApplicationImpl* app);
+                                      mojo::Shell* shell);
 
   // Creates a WindowTreeConnection to service the specified request for
   // a WindowTreeClient. Use this to be embedded in another app.