Streams: commit pull-into descriptors after filling from queue

[MattiasBuelens]

In GHSA-p5g2-876g-95h9, we discovered that in Chromium, a user could run JavaScript code synchronously during ReadableStreamFulfillReadIntoRequest by patching Object.prototype.then, and use this gadget to break some invariants within ReadableByteStreamControllerProcessPullIntoDescriptorsUsingQueue. Fortunately, Deno was unaffected.

The Streams standard has been updated with a proper fix for this case. We now postpone all calls to ReadableByteStreamControllerCommitPullIntoDescriptor until after all pull-into descriptors have been filled up by ReadableByteStreamControllerProcessPullIntoDescriptorsUsingQueue. This way, we won't trigger any patched then() method until the stream is in a stable state.

• Specification change: Commit pull-into descriptors after filling from queue whatwg/streams#1326
• WPT tests: Streams: test whether patched then() sees correct byobRequest web-platform-tests/wpt#48085