Mac: Other approach for IPCing child task_ts.

Based on http://www.foldr.org/~michaelw/log/2009/03/13/ , right now in proof-of-concept quality.

Works for worker processes too and seems more reliable in general. Makes it impossible to call LaunchApp() concurrently though.

Submitting to get perf numbers, will revert.

Review URL: http://codereview.chromium.org/549002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@35883 0039d316-1c4b-4281-b951-d872f2087c98