Add support for retina/high-dpi mac layout test baselines.

For now, we define "mac-retina" as a different port name and treat it as
if it is a different version of the operating system. This will allow us
to have dedicated baselines for it and then fall back to the ML/10.8
baselines for everything else.

This will break if anyone is running 10.7 on a Retina-class device or when
we need to support both 10.8 and 10.9, but is probably good enough for now.

R=ojan@chromium.org
BUG=292886

Review URL: https://chromiumcodereview.appspot.com/23757056

git-svn-id: svn://svn.chromium.org/blink/trunk@157872 bbb929c8-8fbe-4397-9dbb-9b2b20218538