Add Value::Cast.

It's needed for upcasting Persistent<Object> to Persistent<Value> after
handlepocalypse (with Persistent::As or Persistent::Cast).

BUG=
R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/16206014

Patch from Marja Hölttä <marja@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14995 ce2b1a6d-e550-0410-aec6-3dcde31c8c00