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This CL deletes the old content::MediaMetadata types.

BUG=875004

Change-Id: Ia6c5fa76090d2e848261207d6a0dc28e782a4db3
Reviewed-on: https://chromium-review.googlesource.com/c/1241554
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612750}