(WIP) Use generator to generate externs for chrome.fileManagerPrivate API.

ui/file_manager/file_manager/foreground/js/metadata/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': 'byte_reader',
       'includes': ['../../../../compile_js2.gypi'],
     },
     {
@@ skipping 16 matching lines @@
         '<(DEPTH)/ui/file_manager/externs/compiled_resources2.gyp:exif_entry',
         'exif_constants',
         'image_parsers',
         'metadata_parser',
       ],
       'includes': ['../../../../compile_js2.gypi'],
     },
     {
       'target_name': 'external_metadata_provider',
       'dependencies': [
+        '<(EXTERNS_GYP):chrome_extensions',
         '<(EXTERNS_GYP):file_manager_private',
         'metadata_provider',
       ],
       'includes': ['../../../../compile_js2.gypi'],
     },
     {
       'target_name': 'file_system_metadata_provider',
       'dependencies': [
         'metadata_provider',
       ],
@@ skipping 121 matching lines @@
     },
     {
       'target_name': 'thumbnail_model',
       'dependencies': [
         'metadata_model',
       ],
       'includes': ['../../../../compile_js2.gypi'],
     },
   ],
 }