Bug 1574112 [wpt PR 18335] - Implement WebSocketStream, a=testonly

Bug 1574112 [wpt PR 18335] - Implement WebSocketStream, a=testonly

Automatic update from web-platform-tests
Implement WebSocketStream

WebSocketStream is a new API for the WebSocket protocol that uses
streams to support backpressure.

See the explainer for more justification:
https://github.com/ricea/websocketstream-explainer/blob/master/README.md

This CL implements WebSocketStream behind the
experimental-web-platform-features flag.

See the design doc at https://docs.google.com/document/d/1XuxEshh5VYBYm1qRVKordTamCOsR-uGQBCYFcHXP4L0/edit#heading=h.7nki9mck5t64

Bug: 983030
Change-Id: I3002fd52ab151acb579e96a24985988e3fd35053
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1735156
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Adam Rice <ricea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#690263}

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wpt-commits: 23f463a0cc666d24bcb6b50ce2ab8b5deec77881
wpt-pr: 18335