R=rossberg@chromium.org
BUG=v8:1565
TEST=mjsunit/harmony/weakmaps

Review URL: http://codereview.chromium.org/7572013

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8825 ce2b1a6d-e550-0410-aec6-3dcde31c8c00