The two classes now inherit from a common base MediaTrackBase, to
deduplicate the attributes that are shared between the two.
The integer ID from the container is used for each track's id
attribute.
The kind attribute is set to "main" or "translation" according to:
https://dev.w3.org/html5/html-sourcing-inband-tracks/
The label attribute is set to the human-readable name of the track, if
one is present.
The language attribute is set to a BCP 47 language tag, if one can be
parsed successfully.
With this commit, all PlaybackManager can do is autoplay a file from
start to finish, with no pausing or seeking functionality.
All audio playback functionality has been removed from HTMLMediaElement
and HTMLAudioElement in anticipation of PlaybackManager taking that
over, for both audio-only and audio/video.
Resulting in a massive rename across almost everywhere! Alongside the
namespace change, we now have the following names:
* JS::NonnullGCPtr -> GC::Ref
* JS::GCPtr -> GC::Ptr
* JS::HeapFunction -> GC::Function
* JS::CellImpl -> GC::Cell
* JS::Handle -> GC::Root
