helps us decide if we have two ascii strings. We don't care if they
are internalized or not. A few days ago we flipped the meaning of the
internalized bit in INSTANCE_TYPE, and that broke this custom mask.
This CL effects a repair.

BUG=
R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/19514004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@15797 ce2b1a6d-e550-0410-aec6-3dcde31c8c00