This change implements Gerrit protocol for posting try jobs. The
"protocol" is described here:
https://code.google.com/p/chromium/issues/detail?id=366099#c10

CLs for Buildbot:
https://codereview.chromium.org/250983003/
https://codereview.chromium.org/259323003/

R=vadimsh@chromium.org, maruel@chromium.org, kmg@chromium.org
BUG=366099, 364639, 364631

Review URL: https://codereview.chromium.org/254133007

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@267379 0039d316-1c4b-4281-b951-d872f2087c98