Lower the simultaneous rebaseline limit from 200 to 50, in hopes of making the
bot more likely to succeed at rebaselining.

BUG=none
TEST=none
TBR=ojan

Review URL: https://codereview.chromium.org/185173002

git-svn-id: svn://svn.chromium.org/blink/trunk@168233 bbb929c8-8fbe-4397-9dbb-9b2b20218538