This is a reland of a5f3d67b351ca099af4c447c9d91b6d445bf60d9

Original change's description:
> Move ImageDecoder API decoding over to sequenced worker thread.
>
> This moves the actual image decoding into a new ImageDecoderCore
> class which can be used with WTF::SequenceBound to decode images
> on a sequenced worker pool.
>
> This follows the WebCodecs processing model where decoding is not
> supposed to occur on the main thread.
>
> Fixed: 1182435
>
> Change-Id: I22a89d9242110fb17010f1fd178bcb5fc59fb5a1
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2799173
> Reviewed-by: Dan Sanders <sandersd@chromium.org>
> Reviewed-by: Yuta Kitamura <yutak@chromium.org>
> Reviewed-by: Chrome Cunningham <chcunningham@chromium.org>
> Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#869115}

Change-Id: Ic9db3fee402eb59d322dfdf228953b9ea0039f31
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2803918
Auto-Submit: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Yuta Kitamura <yutak@chromium.org>
Reviewed-by: Yuta Kitamura <yutak@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#869409}