Rollback trunk to 3.21.15.

BUG=
R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/23890030

git-svn-id: https://v8.googlecode.com/svn/trunk@16747 ce2b1a6d-e550-0410-aec6-3dcde31c8c00