Clean up port creation in rebaseline.py, baselineoptimizer.

When running 'webkit-patch optimize-baselines', we assumed that creating
the default port on a platform was essentially free, and so we did it
frequently. This is not actually the case on either mac or linux, and
we never actually needed a default port (we could use a fully-specified
port instead).

By making a few changes to only create the ports we needed, this patch
speeds up optimize-baselines by 20-50x.

R=ojan@chromium.org
BUG=410626

Review URL: https://codereview.chromium.org/538823002

git-svn-id: svn://svn.chromium.org/blink/trunk@181395 bbb929c8-8fbe-4397-9dbb-9b2b20218538