Re: [whatwg/streams] Specify WritableStream.[[Transfer]] (#629)

I definitely found it a point in its favor, but wrapping it in a new underlying sink, etc provides its own complications that seem somewhat unappealing when it comes to backpressure signalling.

- For one, it creates an additional queue in the new realm.
- Every time you transfer the already-transferred stream again, you create another stream+queue, chaining to another intermediate stream+queue in every previous realm it's ever been in before chunks reach the original.
- `[[strategySize]]()` only gets to run in the original JS context, so it can't accurately measure any of these other queues which would have to fallback to plain old CountQueuingStrategy instead.
- There's an observable difference between piping through two queues each with a HWM of 1, and one queue with a HWM of 1, so it can't exactly be ignored with an invisible optimization.
- Always setting HWM = 0 wouldn't be an option as that would mean that `writer.ready` is just never fulfilled, even when there is actually no backpressure being exerted by the original underlyingSink.

And there's the question of when, exactly, individual `write()` promises fulfill for the wrapper writablestream.

- Fulfill as soon as the chunk is enqueued into the original realm's WritableStreamController.[[queue]]? This results in no backpressure signalling whatsoever and constantly sending more and more chunks.
- Fulfill when the original `underlyingSink.write()` / `writer.write()` fulfills? The opposite problem: Then you basically ignore the `highWaterMark` of the original writablestream, because the wrapper writablestream will never send a new chunk until the previous one it sent is processed.
- Fulfill when the original `writer.ready` fulfills? What happens when the original WritableStream has HWM = 0, and so it never fulfills at all?

Is it essential that strategySize be invoked synchronously at write time, or can that be changed?

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