Update screen.orientation to follow the latest version of the specs.

Source/modules/screen_orientation/ScreenOrientation.idl

 // 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.
 
+enum OrientationType {
+    "portrait-primary",
+    "portrait-secondary",
+    "landscape-primary",
+    "landscape-secondary"
+};
+
 [
     RuntimeEnabled=ScreenOrientation
 ] partial interface Screen {
-    readonly attribute DOMString orientation;
+    readonly attribute OrientationType orientation;
 
     boolean lockOrientation(DOMString orientation);
 
     void unlockOrientation();
     attribute EventHandler onorientationchange;
 };