[Media Session] Enable API on desktop (all)

Enable Media Session API on desktop for all platforms
since we have features in M73 using this API. This
was previously enable for CrOS and Windows only but
we would like to enable it for consistency on all
desktop platforms.

Intent to Ship: https://groups.google.com/a/chromium.org/d/msg/blink-dev/xIpJ_303phw/6PoGcPIGFQAJ

Desktop PSA: https://groups.google.com/a/chromium.org/d/msg/blink-dev/EQVR9vOrP8o/NkAbV26sDQAJ


Change-Id: I99f9972db64b08f10560b0a318f32a2d1470a3cc
Reviewed-on: https://chromium-review.googlesource.com/c/1432133
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Philip J├Ągenstedt <foolip@chromium.org>
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625687}
8 files changed