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We should update the naming for media metadata artwork images from
"MediaArtwork" to "MediaImage" to match the spec.

Spec URL:
https://wicg.github.io/mediasession/#the-mediaimage-interface

BUG=616411

Review-Url: https://codereview.chromium.org/2394223002
Cr-Commit-Position: refs/heads/master@{#423999}