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This CL adds a heuristic to find associations between video-input and
audio-input devices.

The heuristic considers an association to exist if the label for the
video device (minus the VID:PID suffix, if it exists) appears as a
substring of the audio label.

If zero or more than one association is found then the video device ID
is used as its group ID. This also fixes the problem of having all
video devices with empty group IDs.

We expect this heuristic to have a relatively high false-negative rate
and a low false-positive rate.

Bug: 627793
Change-Id: Iebc9228ca6a8acd5f0e740f3120ef851fecc1cb1
Reviewed-on: https://chromium-review.googlesource.com/883534Reviewed-by: Henrik Boström <hbos@chromium.org>
Reviewed-by: Harald Alvestrand <hta@chromium.org>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532119}