DevTools: [JSDoc] Make event listeners accept non-nullable {!Event}.

Also remove unnecessary casts to {!Event}. This has become possible
due to the recent roll of the Closure compiler.

TBR=vsevik

Review URL: https://codereview.chromium.org/391343003

git-svn-id: svn://svn.chromium.org/blink/trunk@178283 bbb929c8-8fbe-4397-9dbb-9b2b20218538