blink::EventHandler is really hairy and deals with a lot of complex cases that
don't matter for Sky. This CL starts a NewEventHandler and adds support for
PointerEvents there. The NewEventHandler will eventually replace EventHandler
once we've actually migrated over from Mouse+Touch events to PointerEvents.
R=esprehn@chromium.org, ojan@chromium.org, eseidel@chromium.org
Review URL: https://codereview.chromium.org/823873004
