Allow content_browser to access multizone

Just as media_service currently can access the multizone service, we are
adding this capability to the content_browser service.
CastAudioOutputStream runs in content_browser and requires this.

BUG=b:111465269
TEST=Build, play to MZ group through CastAudioOutputStream

Change-Id: I1ffad39d6c61a457f516960228e546462581bef6
Reviewed-on: https://chromium-review.googlesource.com/1139102
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Ken Rockot <rockot@chromium.org>
Commit-Queue: Aidan Wolter <awolter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575845}
1 file changed