Update ongoing promise in async iterator return() method

[MattiasBuelens]

There's an edge case with the current Web IDL bindings for async iterators where manually calling return(); next() could result in the next() promise resolving before the return() promise. The latest version of the Web IDL specification fixes this by updating the "ongoing promise" in the async iterator return() method.

• Web IDL spec change
• Web platform tests (for ReadableStream[@@asyncIterator])