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

url/mojo/gurl.typemap

Index: url/mojo/gurl.typemap
diff --git a/tools/clang/rewrite_to_chrome_style/tests/gen/thing.h b/url/mojo/gurl.typemap
similarity index 55%
copy from tools/clang/rewrite_to_chrome_style/tests/gen/thing.h
copy to url/mojo/gurl.typemap
index 772eaacd59a6b96469422af12a3cc7172a92eca5..adde2550f66091486fd14373760ed61ac9c11f16 100644
--- a/tools/clang/rewrite_to_chrome_style/tests/gen/thing.h
+++ b/url/mojo/gurl.typemap
@@ -1,7 +1,14 @@
-// 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.
-
-namespace blink {
-void genThing();
-}
+// 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.
+
+{
+  "c++": {
+    "url.Url": {
+      "typename": "GURL",
+      "headers": [
+        "url/mojo/url_gurl.h"
+      ]
+    }
+  }
+}