[WebCodecs] By default, only keep the most recently decoded image.

For animates images, keeping all frames can get expensive very fast,
see the attached bug. Originally we expected the built in purging to
handle this for us, but it's set too high to be effective. As such
copy the same optimization <img> uses: When decoding sequentially
only keep the most recent decoded frame.

We add a small escape hatch for out of order decoding so it doesn't
become prohibitively expensive.

In local tests this trade ~8x memory usage for a ~2x CPU usage
increase (1% -> 2%) -- this seems reasonable.

Bug: 1286144
Change-Id: I19f4d719c97d2bd73ca87ce13943626868099b86
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3379609
Reviewed-by: Thomas Guilbert <tguilbert@chromium.org>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/main@{#958244}