[Media Session] Enable API on desktop

Enable Media Session API on desktop for Windows and
Chrome OS as these platforms have features shipping
in M73 that will make use of the API.

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

BUG=922630

Change-Id: I5fbc8acf0667f4e240391eb71350da765682cf43
Reviewed-on: https://chromium-review.googlesource.com/c/1418901
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625051}
6 files changed