Remove pointless uses of EventInit

Setting both bubbles and cancelable to false amounts to the same thing
as using the simpler Event::create, so do that intead.

Review URL: https://codereview.chromium.org/368123002

git-svn-id: svn://svn.chromium.org/blink/trunk@177446 bbb929c8-8fbe-4397-9dbb-9b2b20218538