GitLab

BUG=605198
TEST=add unittest in audio_manager_unittest; unittest doesn't include beamformingdevices case yet as I am still not quite clear about its usage.

tested with https://webrtc.github.io/samples/src/content/devices/input-output/, showing enumerating audio devices in a format of (default) plus other chromeos audio devices.

Review-Url: https://codereview.chromium.org/2079843003
Cr-Commit-Position: refs/heads/master@{#405290}