Simplify StructTraits tests

gpu/ipc/common/traits_test_service.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 gpu.mojom;
 
 import "gpu/ipc/common/mailbox.mojom";
 import "gpu/ipc/common/mailbox_holder.mojom";
 import "gpu/ipc/common/sync_token.mojom";
 
+// All functions on this interface echo their arguments to test StructTraits
+// serialization and deserialization.
 interface TraitsTestService {
+  [Sync]
   EchoMailbox(Mailbox m) => (Mailbox pass);
+
+  [Sync]
   EchoMailboxHolder(MailboxHolder m) => (MailboxHolder pass);
+
+  [Sync]
   EchoSyncToken(SyncToken s) => (SyncToken pass);
 };