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Using an interface makes the usage of MediaImage needlessly more
complicated than needed. A dictionary should keep the feature working
roughly the same way and improve developer's ergonomics.

Spec change: https://github.com/WICG/mediasession/pull/162

BUG=676995
R=zqzhang@chromium.org

Review-Url: https://codereview.chromium.org/2612003002
Cr-Commit-Position: refs/heads/master@{#441962}