Support sync mojo IPCs to and from bindings running on a sequence.

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

Index: mojo/public/cpp/bindings/tests/multiplex_router_unittest.cc
diff --git a/mojo/public/cpp/bindings/tests/multiplex_router_unittest.cc b/mojo/public/cpp/bindings/tests/multiplex_router_unittest.cc
index 0db42db25fc7e4f5a0c78ae02a8a4d31f9bc62a2..a8ceb01ae0232e4bd78183595e7d061eafa5fbee 100644
--- a/mojo/public/cpp/bindings/tests/multiplex_router_unittest.cc
+++ b/mojo/public/cpp/bindings/tests/multiplex_router_unittest.cc
@@ -42,7 +42,12 @@ class MultiplexRouterTest : public testing::Test {
         EmulatePassingEndpointHandle(std::move(endpoint1_), router1_.get());
   }
 
-  void TearDown() override {}
+  void TearDown() override {
+    endpoint1_.reset();
+    endpoint0_.reset();
+    router1_ = nullptr;
+    router0_ = nullptr;
+  }
 
   void PumpMessages() { base::RunLoop().RunUntilIdle(); }