Fix typo in wheel event handler unregistration

This patch fixes a typo in wheel event handler unregistration. It also
brings in a set of wheel event handler tests from
https://bugs.webkit.org/show_bug.cgi?id=77895 which were not landed for
some reason. The tests catch this bug.

TEST=fast/events/wheelevent-handler-count.html

Review URL: https://codereview.chromium.org/195983020

git-svn-id: svn://svn.chromium.org/blink/trunk@169344 bbb929c8-8fbe-4397-9dbb-9b2b20218538