[sensors] Add Permission guard to the generic sensor apis.

third_party/WebKit/Source/modules/permissions/PermissionDescriptor.idl

 // Copyright 2015 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.
 
 enum PermissionName {
     "geolocation",
     "midi",
     "notifications",
     "push",
-    "background-sync"
+    "background-sync",
+    "sensors"
 };
 
 // The PermissionDescriptor dictionary is a base to describe permissions. Some
 // permissions will extend it. The methods reading it will re-parse it depending
 // on the name.
 dictionary PermissionDescriptor {
     required PermissionName name;
 };