Re: [whatwg/streams] Possible spec bug with concurrent byob reads (#1170)

We can fix this by doing `ReadableByteStreamControllerClearPendingPullIntos` in step 10. (We should probably assert that, if there are any pending pull-intos at this step, then there's exactly one and it's an auto-allocated one.)

However, your test case is still invalid. After calling `enqueue()`, `c.byobRequest` is invalidated. You only get a *new* auto-allocated BYOB request after we call `pull()` again. So with the current code, `c.byobRequest` will be `null` and you'll get a "cannot read property 'respond' of null" `TypeError` instead.

We need a slightly more complicated test case to properly demonstrate the bug:
```javascript
promise_test(async () => {
  let count = 0;
  const rs = new ReadableStream({
    type: 'bytes',
    autoAllocateChunkSize: 10,
    pull(c) {
      if (count === 0) {
        c.enqueue(new Uint8Array(10));
      } else {
        c.byobRequest.respond(c.byobRequest.view.byteLength);
      }
      count++;
    }
  });

  const reader1 = rs.getReader();
  console.log(await reader1.read());
  reader1.releaseLock();

  const reader2 = rs.getReader({ mode: 'byob' });
  console.log(await reader2.read(new Uint8Array([1, 2, 3])));

  console.log('test');
}, '#1170');
```
First, we enqueue a chunk to resolve the request from a default reader. The stream will have auto-allocated a BYOB request, but we're not using it. At this point, the stream *should* remove this pull-into descriptor from its list.

Next, we start a read from a BYOB reader. At this point, this *should* have become the first pull-into descriptor, and cause the stream to call `pull()`. But since we never removed the auto-allocated descriptor, we end up in step 11 of [ReadableByteStreamControllerPullInto](https://streams.spec.whatwg.org/commit-snapshots/8bbfbf83ad06a8b9224bbe1cfbcbbbdbdd827a19/#readable-byte-stream-controller-pull-into), where we conclude that we don't need to call pull. Woops. 😛 

With the proposed fix, this scenario works correctly.

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