Forward non-blocking may-begin phase wheel event to renderer main thread.

Currently, MouseWheelEventQueue sets the send_wheel_events_async_ flag
when it receives the ack for kPhaseBegan so that it handles subsequent
wheel events as kEventNonBlocking. The flag is only cleared when the
event queue tries to forward the next kPhaseBegan event. As a result,
kPhaseMayBegin wheel event that follows a previous wheel sequence can be
handled as kEventNonBlocking.

When it happens, InputHandlerProxy::HandleMouseWheel() drops non-blocking
may-begin wheel event, because InputHandlerProxy stores DROP_EVENT as the
prior result for kPhaseEnded or kPhaseCancelled, and reuses the it.

This prevents the scrollbar fade-in working, since the wheel event is not
forwarded to the renderer main thread.

To fix this, InputHandlerProxy::HandleMouseWheel() returns DID_NOT_HANDLE
for all may-begin phase wheel events so that the events are forwarded.

Bug: 40260134
Change-Id: I82394cbd5d30d436650d580b1e0e38e455063140
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6883936
Reviewed-by: Jonathan Ross <jonross@chromium.org>
Commit-Queue: Byungwoo Lee <blee@igalia.com>
Cr-Commit-Position: refs/heads/main@{#1508985}