Mojo C++ bindings: always use MultiplexRouter with InterfacePtr/Binding.

mojo/public/cpp/bindings/lib/interface_ptr_state.h

Index: mojo/public/cpp/bindings/lib/interface_ptr_state.h
diff --git a/mojo/public/cpp/bindings/lib/interface_ptr_state.h b/mojo/public/cpp/bindings/lib/interface_ptr_state.h
index 341901c56d372125336029be1f128e43decc0557..78aa02e17635442ae394a7aa9d0109650fbbab1e 100644
--- a/mojo/public/cpp/bindings/lib/interface_ptr_state.h
+++ b/mojo/public/cpp/bindings/lib/interface_ptr_state.h
@@ -345,8 +345,13 @@ class InterfacePtrState<Interface, true> {
     if (!handle_.is_valid())
       return;
 
-    router_ = new MultiplexRouter(
-        std::move(handle_), MultiplexRouter::MULTI_INTERFACE, true, runner_);
+    MultiplexRouter::Config config =
+        Interface::PassesAssociatedKinds_
+            ? MultiplexRouter::MULTI_INTERFACE
+            : (Interface::HasSyncMethods_
+                   ? MultiplexRouter::SINGLE_INTERFACE_WITH_SYNC_METHODS
+                   : MultiplexRouter::SINGLE_INTERFACE);
+    router_ = new MultiplexRouter(std::move(handle_), config, true, runner_);
     router_->SetMasterInterfaceName(Interface::Name_);
     endpoint_client_.reset(new InterfaceEndpointClient(
         router_->CreateLocalEndpointHandle(kMasterInterfaceId), nullptr,