Mojo C++ bindings: remove the lock in MultiplexRouter if it only serves a single interface.

mojo/public/cpp/bindings/associated_interface_ptr.h

Index: mojo/public/cpp/bindings/associated_interface_ptr.h
diff --git a/mojo/public/cpp/bindings/associated_interface_ptr.h b/mojo/public/cpp/bindings/associated_interface_ptr.h
index 90904512048c371e7784d4eb40c4785b9bf8e679..3bdb319548a379f134ec95e76ec7c6d3d3a0b076 100644
--- a/mojo/public/cpp/bindings/associated_interface_ptr.h
+++ b/mojo/public/cpp/bindings/associated_interface_ptr.h
@@ -231,8 +231,9 @@ template <typename Interface>
 void GetDummyProxyForTesting(AssociatedInterfacePtr<Interface>* proxy) {
   MessagePipe pipe;
   scoped_refptr<internal::MultiplexRouter> router =
-      new internal::MultiplexRouter(false, std::move(pipe.handle0),
-                                    base::ThreadTaskRunnerHandle::Get());
+      new internal::MultiplexRouter(std::move(pipe.handle0),
+                                    internal::MultiplexRouter::MULTI_INTERFACE,
+                                    false, base::ThreadTaskRunnerHandle::Get());
   std::unique_ptr<AssociatedGroup> group = router->CreateAssociatedGroup();
   GetProxy(proxy, group.get());
 }