Mojo JS bindings: flip the default value of use_new_js_bindings to true.

chrome/browser/engagement/BUILD.gn

 # Copyright 2015 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("mojo_bindings") {
   sources = [
     "site_engagement_details.mojom",
   ]
 
   public_deps = [
     "//url/mojo:url_mojom_gurl",
   ]
 
   # TODO(crbug.com/714018): Convert the implementation to use OnceCallback.
   use_once_callback = false
+
+  # TODO(crbug.com/699569): Convert to use the new JS bindings.
+  use_new_js_bindings = false
 }