[whatwg/streams] Cancelling one branch of a tee'd stream never finishes if other branch reads all chunks (#1038)

When teeing a stream, if one branch is cancelled while the other branch is read until the end, then the cancel promise of the first branch never resolves.

## Reproduction case
```javascript
const readable = new ReadableStream({
    start(controller) {
        for (let chunk of ["a", "b", "c"]) {
            controller.enqueue(chunk);
        }
        controller.close();
    }
});
const [leftStream, rightStream] = readable.tee();

(async () => {
    const reader = leftStream.getReader();
    // Cancel the left branch immediately.
    console.log("left cancel start");
    let cancelPromise = reader.cancel();
    // The left branch never finishes cancelling.
    let cancelResult = await cancelPromise;
    console.log("left cancel done", cancelResult); // never reached
})();

(async () => {
    const reader = rightStream.getReader();
    // Read all chunks from the right branch.
    while (true) {
        const {value, done} = await reader.read();
        console.log("right read:", {value, done});
        if (done) break;
    }
    // The right branch has read all the chunks.
    let closedResult = await reader.closed;
    console.log("right closed", closedResult);
})();
```

## Expected results
The left branch should finish cancelling once all chunks have been read from the original stream (as a result of the right branch pulling them).

## Actual results
The left branch never finishes cancelling.

## Investigation
Currently, [the specification for ReadableStreamTee](https://streams.spec.whatwg.org/#readable-stream-tee) only resolves *cancelPromise* when **both** branches are cancelled. However, I believe this needs to happen when either both branches are cancelled, **or** when the last chunk has been pulled (i.e. during the "If done is true" step of *pullAlgorithm*).

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https://github.com/whatwg/streams/issues/1038