Fix auto roll after switching v8 mirror.

BUG=chromium:411835
LOG=n
TBR=tandrii@chromium.org

Review URL: https://codereview.chromium.org/606553003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24216 ce2b1a6d-e550-0410-aec6-3dcde31c8c00