service_constants: Add kOutputNodeVolumeChanged signal

This signal will be monitored by Chrome UI to synchronize
volume when the hardware changes its volume.

BUG=chromium:609458
TEST=None

Change-Id: I9762b60b10eb96e2e4ac7731504a6ce33d604987
Reviewed-on: https://chromium-review.googlesource.com/343326
Commit-Ready: Hsinyu Chao <hychao@chromium.org>
Tested-by: Hsinyu Chao <hychao@chromium.org>
Reviewed-by: Cheng-Yi Chiang <cychiang@chromium.org>
1 file changed