Add IDL and stub implementation for chrome.usersPrivate 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': [
@@ skipping 61 matching lines @@
       'sessions.json',
       'settings_private.idl',
       'signed_in_devices.idl',
       'streams_private.idl',
       'sync_file_system.idl',
       'system_indicator.idl',
       'system_private.json',
       'tab_capture.idl',
       'tabs.json',
       'types.json',
+      'users_private.idl',
       'web_navigation.json',
       # Despite the name, this API does not rely on any
       # WebRTC-specific bits and as such does not belong in
       # the enable_webrtc==0 section below.
       'webrtc_audio_private.idl',
       'webrtc_desktop_capture_private.idl',
       'webrtc_logging_private.idl',
       'webstore_private.json',
       'webstore_widget_private.idl',
       'windows.json',
@@ skipping 72 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',
   },
 }