Reviewed by Darin Adler.

        Move WheelEvent-dispatching logic into WheelEventDispatchMediator.
        https://bugs.webkit.org/show_bug.cgi?id=57642

        Covered by existing tests, no functional change.

        * dom/Event.cpp:
        * dom/Event.h:
        (WebCore::EventDispatchMediator::EventDispatchMediator): Added default
            constructor.
        (WebCore::EventDispatchMediator::event): Inlined.
        (WebCore::EventDispatchMediator::setEvent): Added.
        * dom/EventDispatcher.cpp: Removed dispatchWheelEvent.
        * dom/EventDispatcher.h: Updated decls.
        * dom/Node.cpp:
        (WebCore::Node::dispatchWheelEvent): Changed to use the mediator and
            return a bool, like other dispatch event functions.
        * dom/Node.h: Changed decl.
        * dom/WheelEvent.cpp:
        (WebCore::granularity): Moved from EventDispatcher.
        (WebCore::WheelEventDispatchMediator::WheelEventDisatchMediator): Added.
        (WebCore::WheelEventDispatchMediator::event): Added.
        (WebCore::WheelEventDispatchMediator::dispatchEvent): Added.
        * dom/WheelEvent.h: Updated decls.
        * page/EventHandler.cpp:
        (WebCore::EventHandler::handleWheelEvent): Changed to receive and react
            to the return value.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@83298 268f45cc-cd09-0410-ab3c-d52691b4dbfc