Re: [streams] Add .bytesRead to ReadableByteStream.*Reader (#367)

I think it is a matter of convenience. E.g. it could be convenient, if we allow it, to request a body from one URL, read off the first 200 bytes (the "header" portion), then upload the rest elsewhere. If we disallow such things, then it becomes more manual work for authors, and possibly more memcpys. (Especially since we backed away from writable streams for uploads.)

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https://github.com/whatwg/streams/issues/367#issuecomment-115428031