https://bugs.webkit.org/show_bug.cgi?id=56798

Wrap autorelease pools around calls that can have pathological memory growth on Membuster.  
Only wrap the main runloop because some background threads are not expected to call into 
objc and an autorelease pool could mask bugs.

Reviewed by Simon Fraser.

* Platform/mac/RunLoopMac.mm:
(RunLoop::performWork):
(RunLoop::TimerBase::timerFired):
* WebProcess/WebPage/mac/LayerTreeHostMac.mm:
(WebKit::LayerTreeHostMac::flushPendingLayerChangesRunLoopObserverCallback):



git-svn-id: svn://svn.chromium.org/blink/trunk@81734 bbb929c8-8fbe-4397-9dbb-9b2b20218538