Coalesce wheel events when possible

Combine wheel events if we are getting more than Chromium can handle.
This improves latency and perceived performance when scrolling with
touchpads or fine-grained mouse, on a slow machine or with a debug
build.

Change-Id: Id847c8e7782e155c28067b6051ce92896b68ca7a
Reviewed-by: Michael Brüning <michael.bruning@qt.io>