Correctly expose audio destination for audio resources

Exposing destination exposed the fact that internally we don't distinguish
Audio from Video fetch contexts. This CL fixes that by splitting the Media
fetch context into those two separate ones.

Bug: 520460
Change-Id: Ie7601e4eeea92fd416fdc5dca348cac6520bbf2f
Reviewed-on: https://chromium-review.googlesource.com/846983
Reviewed-by: Mike West <mkwst@chromium.org>
Reviewed-by: Thomas Guilbert <tguilbert@chromium.org>
Commit-Queue: Yoav Weiss <yoav@yoav.ws>
Cr-Commit-Position: refs/heads/master@{#527772}
27 files changed