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

mojo/shell/public/cpp/lib/application_test_base.cc

Index: mojo/shell/public/cpp/lib/application_test_base.cc
diff --git a/mojo/shell/public/cpp/lib/application_test_base.cc b/mojo/shell/public/cpp/lib/application_test_base.cc
index 377220ee0604e9ee0b829506bd5d95c0aa2da860..3d6ad90a093554e4687b20ff006c87d204627dd2 100644
--- a/mojo/shell/public/cpp/lib/application_test_base.cc
+++ b/mojo/shell/public/cpp/lib/application_test_base.cc
@@ -116,7 +116,8 @@ MojoResult RunAllTests(MojoHandle application_request_handle) {
 TestHelper::TestHelper(ApplicationDelegate* delegate)
     : application_impl_(new ApplicationImpl(
           delegate == nullptr ? &default_application_delegate_ : delegate,
-          std::move(g_application_request))) {
+          std::move(g_application_request))),
+      url_(g_url) {
   // Fake application initialization.
   shell::mojom::Application* application = application_impl_.get();
   application->Initialize(std::move(g_shell), g_url, g_id);