Add externs for file_manager.

ui/file_manager/image_loader/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': 'background',
       'variables': {
         'depends': [
           'image_loader.js',
           'cache.js',
           'worker.js',
           'request.js',
         ],
-        'externs': ['<(CLOSURE_DIR)/externs/chrome_send_externs.js'],
+        'externs': [
+          '<(CLOSURE_DIR)/externs/chrome_send_externs.js',
+          '<(CLOSURE_DIR)/externs/chrome_extensions.js',
+          '<(CLOSURE_DIR)/externs/file_manager_private.js',
+        ],
       },
       'includes': [
         '../../../third_party/closure_compiler/compile_js.gypi'
       ],
     }
   ],
 }