Rename [V8DisallowShadowing] to [V8Unforgeable]

Rename [V8DisallowShadowing] to [V8Unforgeable]
https://bugs.webkit.org/show_bug.cgi?id=77599

Reviewed by Adam Barth.

This patch renames [V8DisallowShadowing] to [V8Unforgeable], following the Web IDL
spec (http://dev.w3.org/2006/webapi/WebIDL/#Unforgeable).

No tests. No change in behavior.

* bindings/scripts/CodeGeneratorV8.pm:
(GenerateNormalAttrGetter):
(GenerateSingleBatchedAttribute):
(GenerateImplementation):
* page/DOMWindow.idl:
* page/Location.idl:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@106535 268f45cc-cd09-0410-ab3c-d52691b4dbfc