Replace ApplicationImpl::CreateApplicationConnector() with a standalone helper function.

mojo/public/cpp/application/application_test_base.h

Index: mojo/public/cpp/application/application_test_base.h
diff --git a/mojo/public/cpp/application/application_test_base.h b/mojo/public/cpp/application/application_test_base.h
index 10763be4bf36dcbb561ccb3a162a1f2335cbab4f..eb98b85c2b5df910c63dd77d8b3cb157d1724da0 100644
--- a/mojo/public/cpp/application/application_test_base.h
+++ b/mojo/public/cpp/application/application_test_base.h
@@ -32,6 +32,8 @@ class ApplicationTestBase : public testing::Test {
   ~ApplicationTestBase() override;
 
  protected:
+  // TODO(vtl): Probably should get rid of this. There's really not much reason
+  // to have an |ApplicationImpl|.
   ApplicationImpl* application_impl() { return application_impl_; }
 
   // Get the ApplicationDelegate for the application to be tested.