Add mojo struct traits for GURL so that it can be sent over mojoms.

url/mojo/url.mojom

Index: url/mojo/url.mojom
diff --git a/gpu/command_buffer/common/mailbox.mojom b/url/mojo/url.mojom
similarity index 75%
copy from gpu/command_buffer/common/mailbox.mojom
copy to url/mojo/url.mojom
index 43df62bc82de045c0a154db72a35a517d7b883ee..b97d426d37ddf06dd8ebff195e67391bc1826f11 100644
--- a/gpu/command_buffer/common/mailbox.mojom
+++ b/url/mojo/url.mojom
@@ -1,8 +1,9 @@
-// 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;
-
-[Native]
-struct Mailbox;
+// 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 url.mojom;
+
+struct Url {
+  string url;
+};