[release-tools] Make chromium roll more robust after failing rolls.

[release-tools] Make chromium roll more robust after failing rolls.

Always use the same branch name (the old version leaked
branches). Always try to delete a possibly existing branch
on start-up and also clean up in the end.

NOTRY=true
TBR=tandrii@chromium.org

Review URL: https://codereview.chromium.org/1077633002

Cr-Commit-Position: refs/heads/master@{#27722}