Add struct traits for base::Value.

mojo/common/test_common_custom_types.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 mojo.common.test;
 
 import "mojo/common/file.mojom";
 import "mojo/common/file_path.mojom";
 import "mojo/common/string16.mojom";
 import "mojo/common/time.mojom";
@@ skipping 12 matching lines @@
 
 interface TestTime {
   BounceTime(mojo.common.mojom.Time time) => (mojo.common.mojom.Time time);
   BounceTimeDelta(mojo.common.mojom.TimeDelta time_delta)
       => (mojo.common.mojom.TimeDelta time_delta);
   BounceTimeTicks(mojo.common.mojom.TimeTicks time_ticks)
       => (mojo.common.mojom.TimeTicks time_ticks);
 };
 
 interface TestValue {
+  [Sync]
   BounceDictionaryValue(mojo.common.mojom.DictionaryValue in)
       => (mojo.common.mojom.DictionaryValue out);
+  [Sync]
   BounceListValue(mojo.common.mojom.ListValue in)
       => (mojo.common.mojom.ListValue out);
+  [Sync]
+  BounceValue(mojo.common.mojom.Value? in)
+      => (mojo.common.mojom.Value? out);
 };
 
 interface TestString16 {
   [Sync]
   BounceString16(mojo.common.mojom.String16 in)
       => (mojo.common.mojom.String16 out);
 };
 
 interface TestFile {
   [Sync]
   BounceFile(mojo.common.mojom.File? in)
       => (mojo.common.mojom.File? out);
 };