BufferedChangeEvent members should be nullable

[annevk]

https://w3c.github.io/media-source/#bufferedchangeevent-interface claims its members are non-nullable but that doesn't line up with the constructor.

[chrisn]

Or should they be required?

[annevk]

That's an option, but if this has shipped that might be too late.

[chrisn]

As far as I know, only WebKit has implemented this, but not sure if, for example, Safari differs from WPE

[annevk]

Oh I see, @jyavenard filed #349 which seems related but wouldn't quite solve the issue. I'll dupe this into this issue. @jyavenard should probably decide for WebKit whether we can still change these members to be required.