Remove mouse wheel handler DCHECK

In wheel handling, we assume that if a wheel event is set to
non-blocking it must not be the first in the sequence so the previous
event disposition should be available in mouse_wheel_result_.

In https://crbug.com/1069760 that was found not to be the case. The fix
there was to revert to the previous behavior which checked for whether
that was true (the underlying cause wasn't found) but a DCHECK was left
in. This trips up builds that have DCHECKs enabled as in
https://crbug.com/1156940.

Bug: 1156940,1069760
Change-Id: Ia65b5a0c28c4214492ec0cd9072ff8e5467bfdd7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2580054Reviewed-by: Xida Chen <xidachen@chromium.org>
Commit-Queue: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#835282}

third_party/blink/renderer/platform/widget/input/input_handler_proxy.cc

@@ -896,7 +896,6 @@ InputHandlerProxy::EventDisposition InputHandlerProxy::HandleMouseWheel(
     DCHECK(wheel_event.phase != WebMouseWheelEvent::kPhaseNone ||
            wheel_event.momentum_phase != WebMouseWheelEvent::kPhaseNone);
 
-    DCHECK(mouse_wheel_result_.has_value());
     // TODO(bokan): This should never happen but after changing
     // mouse_event_result_ to a base::Optional, crashes indicate that it does
     // so |if| maintains prior behavior. https://crbug.com/1069760.