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This CL pulls most of the logic out of MediaToolbarButtonController and
puts it into MediaNotificationService which lives as a profile-keyed
service. This allows the media session tracking to be shared between
windows, and makes the MediaToolbarButtonController make more sense.

Bug: 1021369
Change-Id: I03f302cb8b80099df011fa04e47df389c2d5d8f4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1898278Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Takumi Fujimoto <takumif@chromium.org>
Commit-Queue: Tommy Steimel <steimel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712761}