[fetch] Expose body `transmitted` through API surface (#160)

The spec has a concept about the body's stream, length, how much was transmitted, etc.

https://fetch.spec.whatwg.org/#concept-body-transmitted

It doesn't seem like there is currently API surface to access that information.  This would be a really simple and useful way for routinely polling to implement something download progress.

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fetch/issues/160