Add "change" and "removetrack" events to TextTrackList.

LayoutTests/media/track/track-change-event-expected.txt

Index: LayoutTests/media/track/track-change-event-expected.txt
diff --git a/LayoutTests/media/track/track-change-event-expected.txt b/LayoutTests/media/track/track-change-event-expected.txt
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index 0000000000000000000000000000000000000000..e12f0543f015ac97fd0741358438bf9061f533ce
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+++ b/LayoutTests/media/track/track-change-event-expected.txt
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+Tests that the 'change' event is fired when a TextTrack's mode changes.
+
+** Test a TextTrack created with addTextTrack().
+RUN(track = video.addTextTrack('subtitles', 'test', 'en'))
+RUN(track.mode = 'showing')
+EXPECTED (video.textTracks.length == '1') OK
+EVENT(change)
+EXPECTED (event.target == '[object TextTrackList]') OK
+EXPECTED (event instanceof window.Event == 'true') OK
+EXPECTED (event.track == 'undefined') OK
+
+END OF TEST
+