Fix chromium update in chromium roll script.

Needs an extra 'git pull' after a gclient sync, because the solution is
unmanaged now.

TBR=jarin@chromium.org,hinoka@chromium.org

Review URL: https://codereview.chromium.org/529883003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23612 ce2b1a6d-e550-0410-aec6-3dcde31c8c00