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API and motivations described here:
https://github.com/WICG/web-codecs/issues/98

Spec PR here:
https://github.com/WICG/web-codecs/pull/120

Bug: 1141707
Test: new layout tests
Change-Id: I3e53da5961119b4b36ec76666ad1a67cca19b963
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2612684
Commit-Queue: Chrome Cunningham <chcunningham@chromium.org>
Reviewed-by: Thomas Guilbert <tguilbert@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841376}