More long hangs under IOSurfacePool::evict

More long hangs under IOSurfacePool::evict
https://bugs.webkit.org/show_bug.cgi?id=132576
<rdar://problem/16769469>

Reviewed by Simon Fraser.

* platform/graphics/cg/IOSurfacePool.cpp:
(WebCore::IOSurfacePool::evict):
If the pool is too full, we should be evicting in-use surfaces
even if the size of the in-use surfaces has dropped below maximumInUseBytes,
otherwise we can get stuck in an infinite loop with 0 cached surfaces
and 1+ in-use surfaces.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@168308 268f45cc-cd09-0410-ab3c-d52691b4dbfc