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In test scheme while snd-aloop is probed, "Loopback Playback" and "Loopback
Capture" will be shown in system tray as ALSA loopback output and input device.

The input gain could be also scrollable after "Modify mic gainin the system
tray" is enabled in chrome://flags.

BUG=1083465
TEST=Checked "Loopback Playback" and "Loopback Capture" are in system tray when
snd-aloop is probed.
TEST=Checked the scrolls of playback volume and capture gain are both valid.

Change-Id: Iacfc73e76d4ecbb1380b1488b5b7acee4c3c788b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2352129Reviewed-by: Ben Wells <benwells@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Hsinyu Chao <hychao@chromium.org>
Reviewed-by: Bailey Berro <baileyberro@chromium.org>
Reviewed-by: Jenny Zhang <jennyz@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Auto-Submit: Pin-chih Lin <johnylin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801494}