Re: [ServiceWorker] Backpressure on fetch integrated with Streams (#452)

Thanks @annevk for helping remind me what `bodyUsed` was meant for. Namely, something developers can use to avoid try/catch with the convenience methods `text()` etc.

With that in mind I think `bodyUsed`, perhaps with a better name, should be true when any of the following is true:

1. The stream is currently locked
2. The stream is "closed" or "errored" but has Content-Length > 0.

I think that might be enough? That is, I think these are all of the cases where cloning or the convenience methods will fail.

A couple things to note:

- If the stream has been partially read (using `.read()`) but not read to the end, `bodyUsed` should be false, since you could still call `.text()` or clone it and get something useful.
- If `text()` is ever called on a non-empty body, then bodyUsed will be true, since either the stream will be currently locked, or it will have been closed/errored before being unlocked.
- If `text()` is called on an empty stream, then there is no effect. It is safe to call `.text()` on an empty stream as many times as you want.

Does this sound good? Did I miss anything?

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