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

url/mojo/BUILD.gn

+# 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.
+
+import("//mojo/public/tools/bindings/mojom.gni")
+
+mojom("url_mojom") {
+  sources = [
+    "url.mojom",
+  ]
+}
+
+mojom("url_mojom_gurl") {
+  sources = [
+    "url.mojom",
+  ]
+
+  variant = "chromium"
+  typemaps = [ "gurl.typemap" ]
+
+  public_deps = [
+    ":url_mojom",
+  ]
+}
+
+mojom("test_url_mojom_gurl") {
+  sources = [
+    "url_test.mojom",
+  ]
+
+  typemaps = [ "gurl.typemap" ]
+
+  public_deps = [
+    ":url_mojom_gurl",
+  ]
+}