Crashes in ViewGestureController::beginSwipeGesture when swiping in rapid succession

Crashes in ViewGestureController::beginSwipeGesture when swiping in rapid succession
https://bugs.webkit.org/show_bug.cgi?id=136271
<rdar://problem/17923694>

Reviewed by Simon Fraser.

It was possible to get into trackSwipeGesture while another swipe was still
occurring, because the guard against this happening depended on m_pendingSwipeReason
never being set while a swipe was occurring. However, if the very first scroll event
had sufficient magnitude, we would still set m_pendingSwipeReason to InsufficientMagnitude,
and then *never clear it*, leading to a path around the guard against multiple live swipes.
This in turn allowed stale layers in m_liveSwipeLayers, which lead to the crash.

* UIProcess/mac/ViewGestureControllerMac.mm:
(WebKit::ViewGestureController::handleScrollWheelEvent):
Don't unset m_pendingSwipeReason before calling trackSwipeGesture;
trackSwipeGesture will do it itself.

Don't set m_pendingSwipeReason to InsufficientMagnitude
if the event actually *has* sufficient magnitude to start a swipe.

(WebKit::ViewGestureController::trackSwipeGesture):
Assert that we don't have an active gesture while starting a swipe.

Reset m_pendingSwipeReason, because the swipe is no longer pending!

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@172988 268f45cc-cd09-0410-ab3c-d52691b4dbfc