Add dry-run feature to auto roll scripts.

BUG=408523
LOG=n
TBR=jarin@chromium.org

Review URL: https://codereview.chromium.org/529423002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23633 ce2b1a6d-e550-0410-aec6-3dcde31c8c00