Correctly check for AllowHeapAllocation::IsAllowed().

R=adamk@chromium.org, hpayer@chromium.org

Review URL: https://codereview.chromium.org/24302009

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16915 ce2b1a6d-e550-0410-aec6-3dcde31c8c00