This CL changes the type of the buffer used when reading/writing from a
SharedArrayBuffer (SAB) to an ArrayBuffer. SABs were originally used to
clearly denote that the buffer would be observably manipulated by
another "thread" (i.e. the OS as it filled or read the buffer). This had
the risk of allowing identification of the host configuration, by
observing how the buffer was accessed.

The new approach takes an ArrayBuffer, and immediately transfers it. The
given buffer becomes detached (and therefore cannot be used to observe
host operations). The returned buffer references the same memory
allocation as in the inputted one, so there are no copies.

There are 3 different types of changes in this CL:
  - Functional changes, under blink/renderer/modules/native_io
  - Updated tests that use the new interface, where mostly mechanical
    changes were made
  - New tests, that confirm the input buffer is detached, which can be
    found in blink/web_tests/external/wpt/native-io/read_write_async_basic.tentative.https.any.js
    and
    blink/web_tests/external/wpt/native-io/read_write_sync_basic.tentative.https.any.js

Design doc: https://docs.google.com/document/d/1yJv2b5yBhyIj4kOEExEu1s6oZELm2IjywbOrWwNdkuU

Bug: 914488
Change-Id: Iac7b9f4a10e75fee7caa8039d054c7df46f83477
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2784820
Commit-Queue: Emanuel Krivoy <krivoy@google.com>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Reviewed-by: Richard Stotz <rstz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#869786}