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

mash/shelf/shelf_application.h

Index: mash/shelf/shelf_application.h
diff --git a/mash/shelf/shelf_application.h b/mash/shelf/shelf_application.h
index d1ebe297192313b6009c6fe4210c265db9cab859..ab74f2cbc6d1222d5af302e5c40a5bbbf67bd56e 100644
--- a/mash/shelf/shelf_application.h
+++ b/mash/shelf/shelf_application.h
@@ -24,9 +24,8 @@ class ShelfApplication : public mojo::ApplicationDelegate {
 
  private:
   // mojo::ApplicationDelegate:
-  void Initialize(mojo::ApplicationImpl* app) override;
-  bool AcceptConnection(
-      mojo::ApplicationConnection* connection) override;
+  void Initialize(mojo::Shell* shell, const std::string& url,
+                  uint32_t id) override;
 
   mojo::TracingImpl tracing_;