Define writable readyPromise handling

[youennf]

Since this is observable, we need to define when ready promise resolves or not.
By default, follow what implementations currently do and consider chunks have a size equal to zero.

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[alvestrand]

Can you explain what the practical consequences are of having a sizeAlgorithm that returns zero?
I think (having tried to navigate the streams spec) that it will result in a writable stream that is unable to perform backpressure. If this is the property that we desire, we should probably say so explicitly.

[youennf]

I think (having tried to navigate the streams spec) that it will result in a writable stream that is unable to perform backpressure.

Right, we could indeed say so, the writable enqueue operation returns a resolved promise which already achieves this somehow.
The observable consequence of setting a chunk size of 0 is that the writer ready promise will never change (and will always be resolved).

[alvestrand]

Resolution: Add a note after step 7 of the setup about why we're doing 0 and the consequences thereof.