Add the IDL and stub implementation for the chrome.autofillPrivate API.

chrome/common/extensions/api/schemas.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.
 
 {
   'sources': [
     '<@(schema_files)',
   ],
   'variables': {
     'main_schema_files': [
       'accessibility_features.json',
       'accessibility_private.json',
       'activity_log_private.json',
       'audio_modem.idl',
+      'autofill_private.idl',
       'automation.idl',
       'automation_internal.idl',
       'autotest_private.idl',
       'bookmark_manager_private.json',
       'bookmarks.json',
       'braille_display_private.idl',
       'browser.idl',
       'chrome_web_view_internal.json',
       'cloud_print_private.json',
       'command_line_private.json',
@@ skipping 133 matching lines @@
         'schema_files': [
           '<@(webrtc_schema_files)',
         ],
       }],
     ],
     'cc_dir': 'chrome/common/extensions/api',
     'root_namespace': 'extensions::api::%(namespace)s',
     'impl_dir_': 'chrome/browser/extensions/api',
   },
 }