Update builders to delete git/obj dirs when gc --auto breaks.

This allows builders to recover from git corruption.

BUG=chromium:552503
TEST=Unit tests. Trybot run.

Change-Id: I06b85c12a6ef4ac1a9193f6759d18b88d8d4724a
Reviewed-on: https://chromium-review.googlesource.com/311423
Reviewed-by: Richard Barnette <[email protected]>
Reviewed-by: David James <[email protected]>
Commit-Queue: David James <[email protected]>
Trybot-Ready: David James <[email protected]>
Tested-by: David James <[email protected]>
2 files changed