Have the auto-rebaseline tool only rebaseline actual failures on the bots.

Since we spam results into an accumulated directory, the tool would sometimes
get the wrong baseline because of a flaky test having a png from many runs ago.
Instead, only rebaseline results that actually failed in the last run on each
bot.

This bug still exists for rebaselining using rebaseline-expectations and rebaseline,
but I think the proper fix for those is to stop using the accumulated directory
at all, which is blocked on storing the per-run results in an unzipped form.

Even when we stop using the accumulated directory, I think we want to keep this
code for auto-rebaseline because it means the rebaseline will be faster since
it won't try to rebaseline files it doesn't need to.

Review URL: https://chromiumcodereview.appspot.com/20145003

git-svn-id: svn://svn.chromium.org/blink/trunk@154968 bbb929c8-8fbe-4397-9dbb-9b2b20218538