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The default input device reported by Pulse Audio is not really
a default device, but a fallback device.
Actual expected default behavior in Pulse Audio is achieved by
not specifying a device to use and letting Pulse Audio select it.
In this case Pulse first tries to use the device specified by the
PULSE_SOURCE environment variable and, if that fails, tries to use
the fallback device.

There is no API to fetch what device Pulse will actually use by default.
A consequence of this is that the groupId reported by the default
input device will not correspond to the group ID of the actual device
that will be used. This is not ideal, but it is better than the
alternative.

The groupId constrainable property still works fine with nondefault
devices.

Bug: 890010
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: I3473df314f0a36f56094c4986bf8fc9b38265fad
Reviewed-on: https://chromium-review.googlesource.com/c/1256832Reviewed-by: Max Morin <maxmorin@chromium.org>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596170}