[MSE] Resolving Bug 24370

So right now Bug 24370<https://www.w3.org/Bugs/Public/show_bug.cgi?id=24370> is
the only bug left against MSE. That bug triggered the creation of Bug
24563<https://www.w3.org/Bugs/Public/show_bug.cgi?id=24563> which
has resulted in a spirited discussion. Then about 2 weeks ago I had an IM
conversation with Hixie to sort things out and he was still leaning towards
not allowing these attributes to become mutable.

Here is a summary of what I believe his concerns are:
1. kind & language changing during playback was never intended to work in
HTML5. This being possible in out-of-band tracks was just an artifact of
using HTML to specify tracks.
2. Allowing kind & language to change during playback could result in all
sorts of added complexity to the HTML5 spec.
3. There don't appear to be compelling use cases to justify the complexity.
4. If the web application is getting the kind & language data from the
manifest instead of the media file, then why shouldn't the web application
be responsible for selecting appropriate tracks instead of the UA?

After the IM discussion with Hixie, I am starting to wonder myself what
people are actually trying to accomplish with the mutable language and
track attributes. If you look closely at the HTML5 spec there isn't any
existing spec text that talks about what happens if this information
changes after readyState transitions to HAVE_METADATA.

*I'd like the people who care about this particular feature to answer the
following questions so that we can make some progress.*

1. What do you expect the UA to do when kind and/or language is changed on
a xxxTrack object?
2. Why isn't the language & kind data simply placed in the initialization
segments instead of only being in a DASH manifest?
3. Are there real world, compelling examples that require support for
language and/or kind to be able to change during playback?

Without any extra information I doubt we'll be able to resolve this issue.
Right now, I'm leaning towards removing the mutable definitions since there
is no actual UA behavior connected to these attributes changing value.

Aaron