Adds sync message support to Channel-associated interfaces.

ipc/ipc_test.mojom

Index: ipc/ipc_test.mojom
diff --git a/ipc/ipc_test.mojom b/ipc/ipc_test.mojom
index 9c1b30e2a3a51688acc3f8fe1746e37f430856f5..ebe8507011c7cc320f26793c4c62dce9345a4f71 100644
--- a/ipc/ipc_test.mojom
+++ b/ipc/ipc_test.mojom
@@ -5,6 +5,18 @@
 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);
+};