Add IDL and stub implementation for chrome.usersPrivate API.

chrome/common/extensions/api/_api_features.json

 // Copyright (c) 2012 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.
 
 // This features file defines extension APIs implemented under src/chrome.
 // See extensions/common/features/* to understand this file, in particular
 // feature.h, simple_feature.h, and base_feature_provider.h.
 //
 // Note that specifying "web_page", "blessed_web_page", or "all" as a context
 // type will require manually updating chrome/renderer/resources/dispatcher.cc.
@@ skipping 703 matching lines @@
     "contexts": ["blessed_extension"]
   },
   "tts": {
     "dependencies": ["permission:tts"],
     "contexts": ["blessed_extension"]
   },
   "ttsEngine": {
     "dependencies": ["permission:ttsEngine"],
     "contexts": ["blessed_extension"]
   },
+  "usersPrivate": [{
+    "dependencies": ["permission:usersPrivate"],
+    "contexts": ["blessed_extension"]
+  }, {
+    "channel": "trunk",
+    "contexts": ["webui"],
+    "matches": [
+      "chrome://md-settings/*",
+      "chrome://settings/*",
+      "chrome://settings-frame/*"
+    ]
+  }],
   "virtualKeyboardPrivate": {
     "dependencies": ["permission:virtualKeyboardPrivate"],
     "contexts": ["blessed_extension"]
   },
   "wallpaper": {
     "dependencies": ["permission:wallpaper"],
     "contexts": ["blessed_extension"]
   },
   "wallpaperPrivate": {
     "dependencies": ["permission:wallpaperPrivate"],
@@ skipping 42 matching lines @@
     "internal": true,
     "channel": "stable",
     "dependencies": ["permission:webview"],
     "contexts": ["blessed_extension"]
   },
   "windows": {
     "dependencies": ["api:tabs"],
     "contexts": ["blessed_extension"]
   }
 }