Disable webkit/array-iterate-backwards test in --gc-stress mode for turbofan

TBR=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/647743002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24530 ce2b1a6d-e550-0410-aec6-3dcde31c8c00