Enable the "Content hint" attribute of MediaStreamTrack

Also moves a test from internal to public repo.

Bug: chromium:653531
Change-Id: I99224f86fb9abb44784c861b6c2ba0b3c9e020c4
Reviewed-on: https://chromium-review.googlesource.com/1175124
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Commit-Queue: Harald Alvestrand <hta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583585}
3 files changed