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.
Before this change, we were going through the chain of base classes for
each IDL interface object and having them set the prototype to their
prototype.
Instead of doing that, reorder things so that we set the right prototype
immediately in Foo::initialize(), and then don't bother in all the base
class overrides.
This knocks off a ~1% profile item on Speedometer 3.
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
