[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

Committed: https://crrev.com/ec0329e2d6a7444b5280b2c3ac77b65f2be91129
Cr-Commit-Position: refs/heads/master@{#27722}

[Michael Achenbach, 2015-04-09 12:38:21]

machenbach@chromium.org changed reviewers:
+ hablich@chromium.org, tandrii@chromium.org

[Michael Achenbach, 2015-04-09 12:38:58]

PTAL

https://codereview.chromium.org/1077633002/diff/1/tools/release/chromium_roll.py
File tools/release/chromium_roll.py (left):

https://codereview.chromium.org/1077633002/diff/1/tools/release/chromium_roll...
tools/release/chromium_roll.py:48: self["v8_path"] = os.getcwd()
Really all commands have a cwd passed - so no point of actually changing the
directory.

[tandrii(chromium), 2015-04-09 13:10:42]

lgtm

[Michael Achenbach, 2015-04-09 21:35:29]

The CQ bit was checked by machenbach@chromium.org

[commit-bot: I haz the power, 2015-04-09 21:36:04]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1077633002/1

[commit-bot: I haz the power, 2015-04-09 21:37:40]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-09 21:37:53]

Patchset 1 (id:??) landed as
https://crrev.com/ec0329e2d6a7444b5280b2c3ac77b65f2be91129
Cr-Commit-Position: refs/heads/master@{#27722}