Refactoring: Add Event::createCancelable, and use it.

This CL improves code readability.
This is a preparation to remove Event::create(const AtomicString&, bool, bool).

BUG=none
TEST=none; no behavior changes.

Review URL: https://chromiumcodereview.appspot.com/23680002

git-svn-id: svn://svn.chromium.org/blink/trunk@156832 bbb929c8-8fbe-4397-9dbb-9b2b20218538