Trigger repaint on first paint only on pending stylesheet decrement.
Also, since Document::styleResolverChanged() is now just a call to
StyleEngine::resolverChanged(), remove it.
This is in preparation for splitting (style)resolverChanged() into more
descriptive methods on StyleEngine for what is necessary to nuke and
rebuild in the various cases. That, in turn, is in preparation for the
componentized style resolver and asynchronous update of active