Distinction between content-timeline and device-timeline state

[kainino0x]

We already distinguish between content-timeline and device-timeline "steps" in the spec (though not 100% consistently, see #2961).

We should figure out (1) whether, and (2) how, to make the distinction between state on different timelines.
The most complex example is GPUBuffer, which has:

• device timeline state: [[destroyed]]
• state shared by multiple content timelines (when we have multithreading): map state
• state specific to one content timeline: the actual mapped array buffer
• (and of course a content-timeline interface object that can have expando properties and stuff)

[kainino0x]

Another thing that I may as well roll into this issue:

We should find a better name for internal objects like "buffer", "texture", "device", something that's more descriptive like InternalBuffer, Handle, especially if we're going to associate state with it.

[kainino0x]

Have started doing some of this work, in particular for buffer mapping since it's especially complex (#3300). We may or may not fully separate content and device timeline objects for V1 but we'll do what's needed to make the spec clear.

[kainino0x]

#3382 established a pattern for doing this, now we just need to update the rest.