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}

ui/events/blink/input_handler_proxy.cc

@@ -342,8 +342,12 @@ void InputHandlerProxy::DispatchSingleInputEvent(
           std::make_unique<MomentumScrollJankTracker>();
       break;
     case blink::WebGestureEvent::kGestureScrollUpdate:
-      momentum_scroll_jank_tracker_->OnDispatchedInputEvent(
-          event_with_callback.get(), now);
+      // It's possible to get a scroll update without a begin. Ignore these
+      // cases.
+      if (momentum_scroll_jank_tracker_) {
+        momentum_scroll_jank_tracker_->OnDispatchedInputEvent(
+            event_with_callback.get(), now);
+      }
       break;
     case blink::WebGestureEvent::kGestureScrollEnd:
       momentum_scroll_jank_tracker_.reset();