change type of ReadableStream

Previously, we sent ReadableStream<ArrayBuffer>, but specified
only ReadableStream in the API. This change updates the
returned type to ReadableStream<Uint8Array>, and actually
returns a Uint8Array instead of ArrayBuffer. This has the
advantage that new Response() can consume the ReadableStream.

Change-Id: Ie8fddbb9b1c50fb771fe3663a09303e6bb8dbdaf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6203265
Commit-Queue: Dan H <harringtond@chromium.org>
Reviewed-by: Carlos Knippschild <carlosk@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1412394}