Implement gfx::SelectionBound StructTraits

ui/gfx/mojo/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 gfx.mojom;
 
+import "ui/gfx/mojo/selection_bound.mojom";
 import "ui/gfx/mojo/transform.mojom";
 
 // All functions on this interface echo their arguments to test StructTraits
 // serialization and deserialization.
 interface TraitsTestService {
   [Sync]
+  EchoSelectionBound(SelectionBound s) => (SelectionBound pass);
+
+  [Sync]
   EchoTransform(Transform t) => (Transform pass);
 };