Add developer_private.js to closure compiler externs

chrome/browser/resources/extensions/compiled_resources.gyp

 # 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.
 {
   'targets': [
     {
       'target_name': 'extensions',
       'variables': {
         'depends': [
           '../../../../third_party/jstemplate/compiled_resources.gyp:jstemplate',
@@ skipping 15 matching lines @@
           '../../../../ui/webui/resources/js/cr/ui/list_selection_controller.js',
           '../../../../ui/webui/resources/js/cr/ui/list_selection_model.js',
           '../../../../ui/webui/resources/js/cr/ui/overlay.js',
           '../../../../ui/webui/resources/js/event_tracker.js',
           '../../../../ui/webui/resources/js/load_time_data.js',
           '../../../../ui/webui/resources/js/util.js',
         ],
         'externs': [
           '<(CLOSURE_DIR)/externs/chrome_extensions.js',
           '<(CLOSURE_DIR)/externs/chrome_send_externs.js',
+          '<(CLOSURE_DIR)/externs/developer_private.js',
         ],
       },
       'includes': ['../../../../third_party/closure_compiler/compile_js.gypi'],
     }
   ],
 }