Keep svn checkout up-to-date in roll scripts.

BUG=chromium:408523
LOG=n
TBR=jarin@chromium.org
TEST=script_test.py

Review URL: https://codereview.chromium.org/551573002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23755 ce2b1a6d-e550-0410-aec6-3dcde31c8c00