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

Source/modules/screen_orientation/ScreenOrientation.idl

Index: Source/modules/screen_orientation/ScreenOrientation.idl
diff --git a/Source/modules/screen_orientation/ScreenOrientation.idl b/Source/modules/screen_orientation/ScreenOrientation.idl
index ef6cc8c31c71c56f939a327fad14d12fe084f1e1..97a31dc6f1e88ca60e7dbfb89350b3c53460299c 100644
--- a/Source/modules/screen_orientation/ScreenOrientation.idl
+++ b/Source/modules/screen_orientation/ScreenOrientation.idl
@@ -2,10 +2,17 @@
 // 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);