--print_bots arg to 'gcl try' for debugging PRESUBMIT.py changes.
--print_bots arg to 'gcl try' for debugging PRESUBMIT.py changes.

Prints the bots a tryjob would use if sent out.
(Does not send out the job.)

Review URL: http://codereview.chromium.org/9348068

git-svn-id: bd64dd6fa6f3f0ed0c0666d1018379882b742947@121409 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

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