Files app: Move common/js/externs.js to externs/*.

ui/file_manager/compiled_resources2.gyp

 # 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.
 {
   'targets': [
     {
       'target_name': 'file_manager_resources',
       'type': 'none',
       'dependencies': [
         'audio_player/js/compiled_resources2.gyp:*',
         'file_manager/background/js/compiled_resources2.gyp:*',
+        'file_manager/common/js/compiled_resources2.gyp:*',
         'file_manager/foreground/js/compiled_resources2.gyp:*',
         'gallery/js/compiled_resources2.gyp:*',
         'video_player/js/compiled_resources2.gyp:*',
       ],
     },
   ],
 }