Chrome Version: 69.0.3467.0

What steps will reproduce the problem?
(1) Visit a page that uses pointer lock such as https://mdn.github.io/dom-examples/pointer-lock/
(2) While the mouse is locked, use the mouse wheel.

We hit the DCHECK:
[91096:91096:0622/153548.773877:FATAL:mouse_wheel_event_queue.cc(317)] Check failed: event_sent_for_gesture_ack_->event.phase != blink::WebMouseWheelEvent::kPhaseNone || event_sent_for_gesture_ack_->event.momentum_phase != blink::WebMouseWheelEvent::kPhaseNone. 
#0 0x7f623543dc0d base::debug::StackTrace::StackTrace()
#1 0x7f62351864bc base::debug::StackTrace::StackTrace()
#2 0x7f62351f54fa logging::LogMessage::~LogMessage()
#3 0x7f622f3b7308 content::MouseWheelEventQueue::TryForwardNextEventToRenderer()
#4 0x7f622f3b6be2 content::MouseWheelEventQueue::QueueEvent()
#5 0x7f622f3aa81c content::InputRouterImpl::SendWheelEvent()
#6 0x7f622f55a80a content::RenderWidgetHostImpl::ForwardWheelEventWithLatencyInfo()
#7 0x7f622f5964f9 content::RenderWidgetHostViewBase::ProcessMouseWheelEvent()
#8 0x7f622f576a30 content::RenderWidgetHostInputEventRouter::DispatchMouseWheelEvent()
#9 0x7f622f57cd7e content::RenderWidgetHostInputEventRouter::DispatchEventToTarget()
#10 0x7f622f5a7420 content::RenderWidgetTargeter::FoundTarget()
#11 0x7f622f5a69fd content::RenderWidgetTargeter::FindTargetAndDispatch()
#12 0x7f622f57657a content::RenderWidgetHostInputEventRouter::RouteMouseWheelEvent()
#13 0x7f622fa5465e content::RenderWidgetHostViewEventHandler::HandleMouseEventWhileLocked()
#14 0x7f622fa53ca7 content::RenderWidgetHostViewEventHandler::OnMouseEvent()
#15 0x7f622f58cbe5 content::RenderWidgetHostViewAura::OnMouseEvent()
#16 0x7f622a7c4695 ui::EventHandler::OnEvent()
#17 0x7f622a7c2eca ui::EventDispatcher::DispatchEvent()
#18 0x7f622a7c170c ui::EventDispatcher::ProcessEvent()
#19 0x7f622a7c11d4 ui::EventDispatcherDelegate::DispatchEventToTarget()
#20 0x7f622a7c1090 ui::EventDispatcherDelegate::DispatchEvent()
#21 0x7f622a7c5dd9 ui::EventProcessor::OnEventFromSource()
#22 0x7f622a7c611c ui::EventProcessor::OnEventFromSource()
#23 0x7f622a7c84f3 ui::EventSource::DeliverEventToSink()
#24 0x7f622a7c7b55 ui::EventSource::SendEventToSinkFromRewriter()
#25 0x7f622a7c7001 ui::EventSource::SendEventToSink()
#26 0x7f62251bbb5c views::DesktopWindowTreeHostX11::DispatchMouseEvent()
#27 0x7f62251bcb07 views::DesktopWindowTreeHostX11::DispatchEvent()
#28 0x7f6233623a14 ui::PlatformEventSource::DispatchEvent()
#29 0x7f62244c5d31 ui::X11EventSourceGlib::ProcessXEvent()
#30 0x7f62244b5ca5 ui::X11EventSource::ExtractCookieDataDispatchEvent()
#31 0x7f62244b5c14 ui::X11EventSource::DispatchXEvents()
#32 0x7f62244c61d5 ui::(anonymous namespace)::XSourceDispatch()
...