Handle scroll update without begin in MomentumScrollJankTracker

MomentumScrollJankTracker logic assumed that a kGestureScrollUpdate
would always occur betweek kGestureScrollBegin / kGestureScrollEnd.

It seems like we can hit a kGestureScrollUpdate in other cases,
leading to a crash where we tried to use a null object. We now
handle this case.

Bug: 965422
Change-Id: I5538f52ff410e22334e0c0affcea5e987aa00803
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1625822
Commit-Queue: David Bokan <bokan@chromium.org>
Auto-Submit: Eric Karl <ericrk@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663145}
1 file changed