Web Animations: Add an event delegate to allow different forms of event dispatch

Initially this will be used to support CSS Animation events.

BUG=232278

Review URL: https://chromiumcodereview.appspot.com/21156005

git-svn-id: svn://svn.chromium.org/blink/trunk@155389 bbb929c8-8fbe-4397-9dbb-9b2b20218538