The mask to check for an internalized string was incorrectly formed. Hat
tip to Weiliang Lin for discovering the bug.

BUG=v8:3038
LOG=N
R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/108033002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@18265 ce2b1a6d-e550-0410-aec6-3dcde31c8c00