[whatwg/streams] Make pipeThough() non-generic? (#961)

Currently `pipeThrough()` operates generically on **this**. It performs no brand check, and will attempt to call `this.pipeTo()` regardless of whether **this** is a ReadableStream or not.

Blink is currently changing its streams implementation to use IDL, and there's no way to directly express this in IDL. We will need some workaround to pass the tests.

Having `pipeThrough()` be generic complicates the standard (because we have to check if the return value of `pipeTo()` is a Promise), and complicates IDL-based implementations. I think it doesn't provide much value, and the easiest thing to do would be to add a brand check and make it call an abstract PipeTo operation internally.

@domenic, what do you think?

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