ContentFilter should be a ref-counted class
https://bugs.webkit.org/show_bug.cgi?id=108392

Reviewed by David Kilzer.

* loader/MainResourceLoader.h:
(MainResourceLoader): Store a RefPtr to m_contentFilter.
* platform/ContentFilter.h: Inherit from RefCounted<ContentFilter>.
* platform/mac/ContentFilterMac.mm:
(WebCore::ContentFilter::create): Return a PassRefPtr.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@141334 268f45cc-cd09-0410-ab3c-d52691b4dbfc