Make ServiceRegistry (which implements) ApplicationConnection just wrap a ServiceProviderImpl.

mojo/public/cpp/application/application_connection.h

Index: mojo/public/cpp/application/application_connection.h
diff --git a/mojo/public/cpp/application/application_connection.h b/mojo/public/cpp/application/application_connection.h
index 8979d0cf4b81e6a74d4b39ee619e53bc6a67e07b..e665cd4cb87377bf3fb3930248529729798e0124 100644
--- a/mojo/public/cpp/application/application_connection.h
+++ b/mojo/public/cpp/application/application_connection.h
@@ -8,6 +8,7 @@
 #include <string>
 
 #include "mojo/public/cpp/application/lib/interface_factory_connector.h"
+#include "mojo/public/cpp/application/service_provider_impl.h"
 #include "mojo/public/interfaces/application/service_provider.mojom.h"
 
 namespace mojo {
@@ -50,6 +51,8 @@ class ApplicationConnection {
         Interface::Name_);
   }
 
+  virtual ServiceProviderImpl& GetServiceProviderImpl() = 0;
+
   virtual const ConnectionContext& GetConnectionContext() const = 0;
 
   // Returns the URL that was used by the source application to establish a