Leaks builders exit early frequently
https://bugs.webkit.org/show_bug.cgi?id=116355

Reviewed by Stephanie Lewis.

Throttle the number of processes to use, and also bump up the number of crashes/timeouts
we tolerate from 20 to 50.

* BuildSlaveSupport/build.webkit.org-config/master.cfg:
(RunWebKitTests.start):
* Scripts/webkitpy/port/mac.py:
(MacPort.default_child_processes):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@150303 268f45cc-cd09-0410-ab3c-d52691b4dbfc