Migrate wheel events to EventHandlerRegistry

Remove the specialized code for tracking wheel events in favor of the
generic EventHandlerRegistry. Covered by existing tests.

See https://codereview.chromium.org/206603002/ for full review history.

BUG=359566,332793
TEST=fast/events/wheelevent-handler-count.html

Review URL: https://codereview.chromium.org/225823007

git-svn-id: svn://svn.chromium.org/blink/trunk@175485 bbb929c8-8fbe-4397-9dbb-9b2b20218538