Factor clobber out of build/landmines.py

Factor clobber out of build/landmines.py

This patch factors clobbering functionality from build/landmines.py
into build/clobber.py (new file). This will allow us to reuse existing
code in the cr tool.

BUG=493023
Committed: https://crrev.com/af7f4c0efa1389e3989041b3f50a9f647fe2d478
Cr-Commit-Position: refs/heads/master@{#335491}

[petrcermak, 2015-06-22 12:32:16]

petrcermak@chromium.org changed reviewers:
+ jochen@chromium.org

[petrcermak, 2015-06-22 12:32:17]

Hi, please review this patch.

Thanks,
Petr

[jochen (gone - plz use gerrit), 2015-06-22 12:34:02]

lgtm

[petrcermak, 2015-06-22 12:34:34]

The CQ bit was checked by petrcermak@chromium.org

[commit-bot: I haz the power, 2015-06-22 12:34:41]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1199933004/1

[commit-bot: I haz the power, 2015-06-22 12:42:08]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-06-22 12:42:59]

Patchset 1 (id:??) landed as
https://crrev.com/af7f4c0efa1389e3989041b3f50a9f647fe2d478
Cr-Commit-Position: refs/heads/master@{#335491}