TEST=valgrind reports 0 bytes definitely lost for cctest/test-api/RunTwoIsolatesOnSingleThread

Review URL: http://codereview.chromium.org/7621064

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@8968 ce2b1a6d-e550-0410-aec6-3dcde31c8c00