WebProcess is often killed due to being unresponsive on the bots

Reviewed by Alexey Proskuryakov.

Blind attempt to improve flakiness on the bots by reducing the number
of crashes due to an unresponsive process.  This helped on my personal machine
under stress conditions.  I don't have enough cores to hit the problem
under normal conditions.

* WebKitTestRunner/TestController.cpp:
(WTR):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@118860 268f45cc-cd09-0410-ab3c-d52691b4dbfc