Revert of 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 09d768e5abd15e4747cd094554e3851612c1ed64..341901c56d372125336029be1f128e43decc0557 100644
--- a/mojo/public/cpp/bindings/lib/interface_ptr_state.h
+++ b/mojo/public/cpp/bindings/lib/interface_ptr_state.h
@@ -345,11 +345,8 @@
     if (!handle_.is_valid())
       return;
 
-    router_ = new MultiplexRouter(std::move(handle_),
-                                  Interface::PassesAssociatedKinds_
-                                      ? MultiplexRouter::MULTI_INTERFACE
-                                      : MultiplexRouter::SINGLE_INTERFACE,
-                                  true, runner_);
+    router_ = new MultiplexRouter(
+        std::move(handle_), MultiplexRouter::MULTI_INTERFACE, true, runner_);
     router_->SetMasterInterfaceName(Interface::Name_);
     endpoint_client_.reset(new InterfaceEndpointClient(
         router_->CreateLocalEndpointHandle(kMasterInterfaceId), nullptr,