Add FlushForTesting to InterfacePtr and Binding.

mojo/public/cpp/bindings/lib/control_message_handler.cc

Index: mojo/public/cpp/bindings/lib/control_message_handler.cc
diff --git a/mojo/public/cpp/bindings/lib/control_message_handler.cc b/mojo/public/cpp/bindings/lib/control_message_handler.cc
index b9bf7eb54ed5aaf96078f62181e58a42af7aa4ca..462e3e08dc51ca80466cc32cd772b2a84eb914c7 100644
--- a/mojo/public/cpp/bindings/lib/control_message_handler.cc
+++ b/mojo/public/cpp/bindings/lib/control_message_handler.cc
@@ -61,6 +61,8 @@ bool ControlMessageHandler::Run(Message* message,
     output->set_query_version_result(
         interface_control::QueryVersionResult::New());
     output->get_query_version_result()->version = interface_version_;
+  } else if (input.is_flush_for_testing()) {
+    output.reset();
   } else {
     output.reset();
   }