swift format is only available on mac hosts.
Swift formatting is disabled by default. It will be
enabled by default on mac host in another CL.

Change-Id: Id603be203edd44b6419d545027c249111abe1baf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3550197Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Olivier Robin <olivierrobin@chromium.org>