Simplify StructTraits tests

gpu/ipc/common/traits_test_service.mojom

Index: gpu/ipc/common/traits_test_service.mojom
diff --git a/gpu/ipc/common/traits_test_service.mojom b/gpu/ipc/common/traits_test_service.mojom
index 4097e5659d0c33048057c3acb049a53f907811bc..f19bf949c6a294a903b4f479a2172e69c0fe6cda 100644
--- a/gpu/ipc/common/traits_test_service.mojom
+++ b/gpu/ipc/common/traits_test_service.mojom
@@ -8,8 +8,15 @@ 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);
 };