Replace calls to Document::createEvent() with Event::create calls

This avoids a funny-looking static_pointer_cast and eliminates the only
caller of createEvent() in core (it should only be called by the bindings).

R=eseidel

Review URL: https://codereview.chromium.org/37683002

git-svn-id: svn://svn.chromium.org/blink/trunk@160372 bbb929c8-8fbe-4397-9dbb-9b2b20218538