WebKit should throttle memory pressure notifications in proportion to handler time

WebKit should throttle memory pressure notifications in proportion to handler time
https://bugs.webkit.org/show_bug.cgi?id=82674

Reviewed by Geoffrey Garen.

Changed the MemoryPressureHandler hold off timer to start timing after
respondToMemoryPressure runs.  The delay time is now 20 times longer than the
time it took for respondToMemoryPressure to run with a minimum of 5 seconds.
This throttles the response to low memory events in the extreme case where
we are spending most of our time paging / swapping.
This is a Mac only change.

No additional tests. This passes existing test and was verified using
manual tests on a small memory system with many websites open.

* platform/mac/MemoryPressureHandlerMac.mm:
(WebCore):
(WebCore::MemoryPressureHandler::holdOff):
(WebCore::MemoryPressureHandler::respondToMemoryPressure):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@112910 268f45cc-cd09-0410-ab3c-d52691b4dbfc