Adds sync message support to Channel-associated interfaces.

ipc/ipc_test.mojom

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 module IPC.mojom;
 
 interface SimpleTestDriver {
-  ExpectString(string str);
+  ExpectValue(int32 value);
+
+  [Sync]
+  GetExpectedValue() => (int32 value);
+
+  [Sync]
+  RequestValue() => (int32 value);
+
   RequestQuit() => ();
 };
+
+interface SimpleTestClient {
+  [Sync]
+  RequestValue() => (int32 value);
+};