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This CL moves the MediaMetadata and MediaImage mojo types
from Blink to the Media Session service along with their
mapped types in content.

This is the first of four parts.

BUG=875004

Change-Id: Ie862229bbeb474a60ccc4d2f82d8c4e1f8017b74
Reviewed-on: https://chromium-review.googlesource.com/c/1227401Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605481}