TrackEvent.track should be (VideoTrack or AudioTrack or TextTrack)?

Track attribute uses object type, but we should use
(VideoTrack or AudioTrack or TextTrak)? as the spec says[1].

[1] https://html.spec.whatwg.org/multipage/embedded-content.html#the-trackevent-interface

BUG=430337

Review URL: https://codereview.chromium.org/690923004

git-svn-id: svn://svn.chromium.org/blink/trunk@185104 bbb929c8-8fbe-4397-9dbb-9b2b20218538