This avoid leaking memory because optimized code may hold on to maps
from different tabs otherwise.

BUG=v8:1823, 102895
Review URL: http://codereview.chromium.org/8892002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@10257 ce2b1a6d-e550-0410-aec6-3dcde31c8c00