Mojo C++ bindings: introduce MultiplexRouter and related classes.

mojo/public/cpp/bindings/tests/validation_unittest.cc

Index: mojo/public/cpp/bindings/tests/validation_unittest.cc
diff --git a/mojo/public/cpp/bindings/tests/validation_unittest.cc b/mojo/public/cpp/bindings/tests/validation_unittest.cc
index fc536c2af5022a6fd33a455dac2a7f02e017eca0..2fd98aa910f2db965406bad9514b3439071ba740 100644
--- a/mojo/public/cpp/bindings/tests/validation_unittest.cc
+++ b/mojo/public/cpp/bindings/tests/validation_unittest.cc
@@ -221,7 +221,9 @@ class ValidationIntegrationTest : public ValidationTest {
    public:
     TestMessageReceiver(ValidationIntegrationTest* owner,
                         ScopedMessagePipeHandle handle)
-        : owner_(owner), connector_(handle.Pass()) {
+        : owner_(owner),
+          connector_(handle.Pass(),
+                     mojo::internal::Connector::SINGLE_THREADED_SEND) {
       connector_.set_enforce_errors_from_incoming_receiver(false);
     }
     ~TestMessageReceiver() override {}