[Media Session] Add SpecMediaMetadata

This is a follow up from https://crrev.com/c/1334661
which splits out spec metadata between Blink/Content
with its own SpecMediaMetadata.

BUG=892771

Change-Id: I48825f4c97e526888d5a9840cdcc129e2eed11e9
Reviewed-on: https://chromium-review.googlesource.com/c/1423400
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#628192}
9 files changed