Dropping cues when playbackRate != 1.0

There's section of the HTML5 CR spec saying:
> Similarly, when the playback rate is not exactly 1.0, hardware,
> software, or format limitations can cause video frames to be dropped
> and audio to be choppy or muted.

I think this should also apply to text track cues, something like:
> Similarly, when the playback rate is not exactly 1.0, hardware,
> software, or format limitations can cause video frames *and text cues*
> to be dropped and audio to be choppy or muted.

When playing at non-standard speeds, an efficient implementation may
want to skip portions of the file, which could mean skipping cues.