Add a --wait-for-trybots flag that will have the script look for
a previously created roll and poll the trybot status until at least
one trybot has failed or all have compelted successfully.
BUG=chromium:433305
TESTED=Ran:
tools/autoroller/roll_webrtc_in_chromium.py --chromium-checkout /ssd/chrome/src --verbose
tools/autoroller/roll_webrtc_in_chromium.py --chromium-checkout /ssd/chrome/src --verbose --wait-for-trybots
R=tommi@webrtc.org
Review URL: https://webrtc-codereview.appspot.com/45659004
Cr-Commit-Position: refs/heads/master@{#8718}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8718 4adac7df-926f-26a2-2b94-8c16560cd09d
c449c20f4d