Added support for enabling GCD, or not, at build time.

remoting/remoting_webapp.gypi

 # Copyright 2014 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.
 #
 # File included in remoting_webapp_* targets in remoting_client.gypi
 
 {
   'type': 'none',
   'variables': {
     'extra_files%': [],
@@ skipping 119 matching lines @@
         '<(version_full)',
         '<(output_dir)',
         '<(zip_path)',
         'webapp/crd/manifest.json.jinja2',
         '<(webapp_type)',
         '<@(generated_html_files)',
         '<@(remoting_webapp_crd_files)',
         '<@(extra_files)',
         '--locales_listfile',
         '<(dr_webapp_locales_listfile)',
+        '--use_gcd',
+        '<(remoting_use_gcd)',
       ],
     },
   ],
 }