Crashes seen in wheel event handling
https://bugs.webkit.org/show_bug.cgi?id=135102

Reviewed by Beth Dakin.

Speculative fix based on guesses about what could be crashing.
The crash seems to be calling ref on an event target, and my guess is that this
has something to do with latching.

* page/EventHandler.cpp:
(WebCore::EventHandler::platformPrepareForWheelEvents): Updated argument types.
(WebCore::EventHandler::handleWheelEvent): Refactored a little and made some local
variables use RefPtr instead of raw pointers. Also added some comments.

* page/EventHandler.h: Changed argument types to RefPtr.

* page/mac/EventHandlerMac.mm:
(WebCore::EventHandler::platformPrepareForWheelEvents): Updated argument types.
Also added a FIXME.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@171283 268f45cc-cd09-0410-ab3c-d52691b4dbfc