[iOS WebKit2] IOSurfacePool should force CA to actually garbage collect surfaces.

[iOS WebKit2] IOSurfacePool should force CA to actually garbage collect surfaces.
<https://webkit.org/b/132065>
<rdar://problem/16110687>

Add a platformGarbageCollectNow() helper function to IOSurfacePool that
triggers a sweep of the IOSurfaces. Call this from collectionTimerFired()
and discardAllSurfaces().

This lets us drop all otherwise-unused 420f surfaces on memory pressure.

Reviewed by Tim Horton.

* WebCore.xcodeproj/project.pbxproj:
* platform/graphics/cg/IOSurfacePool.cpp:
(WebCore::IOSurfacePool::collectionTimerFired):
(WebCore::IOSurfacePool::discardAllSurfaces):
* platform/graphics/cg/IOSurfacePool.h:
* platform/graphics/cocoa/IOSurfacePoolCocoa.mm: Added.
(WebCore::IOSurfacePool::platformGarbageCollectNow):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@167717 268f45cc-cd09-0410-ab3c-d52691b4dbfc