Fix stepping in for-loops.

R=ulan@chromium.org
BUG=v8:3634
LOG=N

Review URL: https://codereview.chromium.org/682413004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25231 ce2b1a6d-e550-0410-aec6-3dcde31c8c00