Convert Swarm bot on OSX to be launched by launchd.
Convert Swarm bot on OSX to be launched by launchd.

This should fix GUI tests issues that were occuring because the bot was started
via a cron job. Starting via a cronjob makes Aqua unavailable, causing most
tests to fail.

Then the status of the swarm slave can be looked at with
  launchctl list | grep swarm

R=csharp@chromium.org
BUG=

Review URL: https://chromiumcodereview.appspot.com/15679020

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@204288 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build