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This change adds an action to the MediaSessionAction enum for signaling
if audio output device switching is possible for that session. A session
supports switching audio devices if and only if the session has at
least one player and all of the players support audio device switching.

When the audio output device cannot be changed, the Global Media
Controls UI should not display a device picker. This change is a part
of a series of changes for appropriately hiding this UI.

Bug: 1120620
Change-Id: If03ef110ff32fa0db3463c63a6a1730da60d2278
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2369440
Commit-Queue: Noah Rose Ledesma <noahrose@google.com>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Fabrice de Gans-Riberi <fdegans@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Reviewed-by: Tommy Steimel <steimel@chromium.org>
Reviewed-by: Becca Hughes <beccahughes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#802666}