issues with a raw this pointer and GC.

Fix issue with FlattenString where we may keep a StringShape
alive across a GC.
Review URL: http://codereview.chromium.org/43020

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@1481 ce2b1a6d-e550-0410-aec6-3dcde31c8c00