Wheel events don't latch to inner scrollable elements

Wheel events don't latch to inner scrollable elements
https://bugs.webkit.org/show_bug.cgi?id=128225

Reviewed by Beth Dakin.

* page/EventHandler.cpp:
(WebCore::EventHandler::handleWheelEvent): Identify the case
where we have hit the end of a scroll, and treat that as a
valid 'handled' case. If the scroll event is just starting,
treat end-of-scroll as unhandled so the parent element can
handle things.
* page/WheelEventDeltaTracker.h:
(WebCore::WheelEventDeltaTracker::isFirstWheelEvent): Added.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@163558 268f45cc-cd09-0410-ab3c-d52691b4dbfc