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

mojo/shell/fetcher/about_fetcher_unittest.cc

Index: mojo/shell/fetcher/about_fetcher_unittest.cc
diff --git a/mojo/shell/fetcher/about_fetcher_unittest.cc b/mojo/shell/fetcher/about_fetcher_unittest.cc
index 84878e9d48f31c8b9f7998a308eb248d6607b7d1..a2805259eef9b2921f4c3a9864545bbd6bec3b34 100644
--- a/mojo/shell/fetcher/about_fetcher_unittest.cc
+++ b/mojo/shell/fetcher/about_fetcher_unittest.cc
@@ -42,7 +42,7 @@ class TestContentHandler : public ApplicationDelegate,
 
  private:
   // Overridden from ApplicationDelegate:
-  void Initialize(ApplicationImpl* app) override {}
+  void Initialize(Shell* shell, const std::string& url, uint32_t id) override {}
   bool AcceptConnection(ApplicationConnection* connection) override {
     connection->AddService<mojom::ContentHandler>(this);
     return true;