This will replace the hack in commit-queue/verification/presubmit_shim, and will
be used on the presubmit trybot.

R=maruel@chromium.org
BUG=

Review URL: https://codereview.chromium.org/12481002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@186922 0039d316-1c4b-4281-b951-d872f2087c98