[NowPlaying] Give MediaSession playback state to MPNowPlayingInfoCenter

This CL adds a NowPlayingInfoCenterDelegate class that interacts with
Mac OS's MPNowPlayingInfoCenter API. This also adds a new media session
observer that uses the NowPlayingInfoCenterDelegate to update the "Now
Playing" playback state.

This CL only adds playback state data, which allows the Mac OS to
properly decide whether Chrome or another app should receive the media
keys.

Bug: 936513, 930180
Change-Id: Ic4a62905e590bc8185d7d6ce7beb1e639d1408d3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1492778
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Tommy Steimel <steimel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641422}