Add use counter for <audio>.setSinkId()

Getting a sense for usage of this API as part of considering wether the
MediaDevices API is the right place to describe sink channel configuration

See
https://docs.google.com/document/d/1to7llKOyNZxirnpCahsslKnUazQZnT2mEVfRaWbCss0/edit#heading=h.rqmkwsts4vfx

Change-Id: I214c768e46809f8621d689778870bea5ca797454
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2822324


Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#872082}