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This CL moves any existing uses of content::MediaMetadata
to the new media_session::MediaMetadata.

BUG=875004

Change-Id: If4b2f889ffb2264daa2bc2f5d9403e9e18586856
Reviewed-on: https://chromium-review.googlesource.com/c/1241572
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612704}