Limit the number of tests that we auto rebaseline at once.

Reitveld's uploader stinks and dies halfway through large rebaselines.
This is a short-term solution to keep the bot running.

R=dpranke@chromium.org, eae@chromium.org

Review URL: https://codereview.chromium.org/20068004

git-svn-id: svn://svn.chromium.org/blink/trunk@154801 bbb929c8-8fbe-4397-9dbb-9b2b20218538