Allow Object::InternalFieldCount and Object::GetAlignedPointerFromInternalField to be called from Persistent classes

R=svenpanne@chromium.org

BUG=

Review URL: https://codereview.chromium.org/177343002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@19740 ce2b1a6d-e550-0410-aec6-3dcde31c8c00