CachedResourceLoader hoards URLs indefinitely for no good reason.

CachedResourceLoader hoards URLs indefinitely for no good reason.
<https://webkit.org/b/132102>
<rdar://problem/16708265>

Since we don't care about CachedResourceLoader::m_validatedURL's after
the Document has finished dispatching its initial load event, clear the
set at that point, and don't add any new URLs to it.

Reviewed by Anders Carlsson.

* dom/Document.cpp:
(WebCore::Document::dispatchWindowLoadEvent):
* loader/cache/CachedResourceLoader.cpp:
(WebCore::CachedResourceLoader::requestResource):
(WebCore::CachedResourceLoader::documentDidFinishLoadEvent):
* loader/cache/CachedResourceLoader.h:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@167743 268f45cc-cd09-0410-ab3c-d52691b4dbfc