Teach rebaseline log server to execute commands on behalf of rebaseline-o-matic

R=ojan@chromium.org
BUG=425069

Review URL: https://codereview.chromium.org/647763003

git-svn-id: svn://svn.chromium.org/blink/trunk@184694 bbb929c8-8fbe-4397-9dbb-9b2b20218538