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

components/font_service/font_service_app.h

Index: components/font_service/font_service_app.h
diff --git a/components/font_service/font_service_app.h b/components/font_service/font_service_app.h
index abdd0f1a84061b3e47369e81348d182350a2e535..914eb08ed9c90c5a9042d1701cc9123cb8c505a4 100644
--- a/components/font_service/font_service_app.h
+++ b/components/font_service/font_service_app.h
@@ -26,7 +26,8 @@ class FontServiceApp : public mojo::ApplicationDelegate,
 
  private:
   // ApplicationDelegate:
-  void Initialize(mojo::ApplicationImpl* app) override;
+  void Initialize(mojo::Shell* shell, const std::string& url,
+                  uint32_t id) override;
   bool AcceptConnection(
       mojo::ApplicationConnection* connection) override;