Memory cache live resources repeatedly purged during painting

Memory cache live resources repeatedly purged during painting
https://bugs.webkit.org/show_bug.cgi?id=144104
<rdar://problem/20667695>

Reviewed by Chris Dumez.

On some PLT pages (like nytimes.com) we get into state where painting repeatedly purges live bitmaps.
This slows down page loads significantly.

This might have regressed because improvements in page caching keep more pages and so resources 'live'.

With this path we do all regular cache pruning asynchronously. If memory is really critical
the low memory handling code will still prune synchronously.

* loader/cache/CachedResource.cpp:
(WebCore::CachedResource::removeClient):
(WebCore::CachedResource::didAccessDecodedData):

    prune() -> pruneSoon()

* loader/cache/MemoryCache.cpp:

    Decrease the pruning size target from 0.95 to 0.8 so we don't need to prune so often.

(WebCore::MemoryCache::needsPruning):

    Factor into a function.

(WebCore::MemoryCache::prune):
(WebCore::MemoryCache::pruneSoon):

    Prune asynchronously.

* loader/cache/MemoryCache.h:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@183194 268f45cc-cd09-0410-ab3c-d52691b4dbfc