[whatwg/streams] Permitting transferable stream optimisation (#1124)

Chrome implements an optimisation for native streams that effectively allows the underlying source to move to the new thread when they are transferred to a worker. This is a really good optimisation, but a strict reading of the standard doesn't permit it.

The problem is that the standard requires a transferred stream to stop working when the original source realm is destroyed. However, when Chrome optimises a native stream transfer it becomes completely detached from the original source realm and so it keeps working.

We did have a prototype which errored the stream when the original context was destroyed, but it didn't actually match the observed behaviour of transferred streams, which is more complex. Precisely emulating the real behaviour does not seem achievable.

We'd like to relax the standard language in a way that makes Chrome's optimisation conformant.

This is related to #1063, but fixing that wouldn't fix this.

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