Rename chromium-mac* ports to mac*

Also renames the ChromiumMacPort class to MacPort.

A later change will update the relevant LayoutTest platform directories
accordingly.

BUG=231188
R=ojan@chromium.org

Review URL: https://codereview.chromium.org/18084009

git-svn-id: svn://svn.chromium.org/blink/trunk@153309 bbb929c8-8fbe-4397-9dbb-9b2b20218538