Tolerate unstarted builds in rebaseline server.

Currently server.py --rietveld-issue will crash if there are any
unstarted trybots on the latest patch. Instead of crashing, warn
that the bot hasn't started yet.

Review URL: https://codereview.chromium.org/758793003
Don't upload metrics we don't want to track.

BUG=skia:

Review URL: https://codereview.chromium.org/758853004
remove unnecessary save/clip/restore

BUG=skia:

Review URL: https://codereview.chromium.org/785493003