Mojo JS bindings: add support to generate "dual-mode" code.

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_gurl") {
   sources = [
     "url.mojom",
   ]
 
   # TODO(crbug.com/699569): Convert to use the new JS bindings.
-  use_new_js_bindings = false
+  js_bindings_mode = "both"
 }
 
 mojom("url_mojom_origin") {
   sources = [
     "origin.mojom",
   ]
 
   public_deps = [
     ":url_mojom_gurl",
   ]
 
   # TODO(crbug.com/699569): Convert to use the new JS bindings.
-  use_new_js_bindings = false
+  js_bindings_mode = "both"
 }
 
 mojom("test_url_mojom_gurl") {
   sources = [
     "url_test.mojom",
   ]
 
   public_deps = [
     ":url_mojom_gurl",
     ":url_mojom_origin",
   ]
 }