Use PlatformWheelEvent::phase() to determine if a scroll gesture begins or ends

Use PlatformWheelEvent::phase() to determine if a scroll gesture begins or ends
https://bugs.webkit.org/show_bug.cgi?id=77127

Reviewed by Beth Dakin.

* platform/mac/ScrollAnimatorMac.h:
(ScrollAnimatorMac):
Remove handleGestureEvent.

* platform/mac/ScrollAnimatorMac.mm:
(WebCore::ScrollAnimatorMac::handleWheelEvent):
Look at the event phase to determine when to call didBeginGesture and didEndGesture.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@106026 268f45cc-cd09-0410-ab3c-d52691b4dbfc