Fix previous backmerge (3.21.18.10 / r17958)

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/80753004

git-svn-id: https://v8.googlecode.com/svn/branches/3.21@17974 ce2b1a6d-e550-0410-aec6-3dcde31c8c00