[whatwg/streams] TransformStream does not propagate the argument passed to cancel (#768)

I just want to check that this is the expected behaviour.

```javascript
const ts = new TransformStream();
const closedPromise = ts.writable.getWriter().closed;
ts.readable.cancel(new Error('hay'));
closedPromise.catch(e => console.log(e.message));  // logs 'Readable side canceled'
```

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