Add IDL and stub implementation for chrome.usersPrivate API.

chrome/common/extensions/api/_permission_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 permissions for 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.
 
 // To add a new whitelisted ID, SHA-1 it and force it to uppercase. In Bash:
@@ skipping 945 matching lines @@
     "extension_types": ["extension", "legacy_packaged_app"]
   },
   "tts": {
     "channel": "stable",
     "extension_types": ["extension", "legacy_packaged_app", "platform_app"]
   },
   "ttsEngine": {
     "channel": "stable",
     "extension_types": ["extension", "legacy_packaged_app"]
   },
+  "usersPrivate": {
+    "channel": "trunk",
+    "extension_types": ["extension", "platform_app"],
+    "location": "component"
+  },
   "wallpaper": {
     "channel": "stable",
     "extension_types": ["platform_app", "extension"],
     "platforms": ["chromeos"]
   },
   "wallpaperPrivate": {
     "channel": "stable",
     "extension_types": ["platform_app"],
     "location": "component",
     "platforms": ["chromeos"]
@@ skipping 48 matching lines @@
     "channel": "stable",
     "extension_types": ["platform_app"],
     "platforms": ["chromeos"],
     "whitelist": [
       "A948368FC53BE437A55FEB414106E207925482F5",  // Files app
       "8C726564C6DBE7380BAB388DE6CC7EC93EB44B06",  // Video player
       "5065C83E84321221675D20FBE4DC43C655CC8C6F"   // Files app tests
     ]
   }
 }