Introduce Event::isWheelEvent() and use it to prevent a bad cast.

This is a fix similar to https://codereview.chromium.org/25837002/.

BUG=303232

Review URL: https://codereview.chromium.org/25859002

git-svn-id: svn://svn.chromium.org/blink/trunk@158890 bbb929c8-8fbe-4397-9dbb-9b2b20218538