Optimize baselines in parallel.

This is by far the slowest part of doing rebaselines.
We can do everything but the SCM commands in parallel.
Like we do for the other commands, save off the SCM commands
and do them all at the end.

Review URL: https://codereview.chromium.org/308793004

git-svn-id: svn://svn.chromium.org/blink/trunk@175191 bbb929c8-8fbe-4397-9dbb-9b2b20218538