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The video-capture subsystem currently does not support the concept
of group IDs, which exists in the MediaStreams spec.
To support video group IDs, we use a heuristic that finds
associations between video and audio devices.
Prior to this CL, this heuristic was used to patch results
requested by the MediaDevices.enumerateDevices() JavaScript function,
but low-level enumerations used by other parts of Chrome
may have kept video group IDs empty.
To ensure that device associations are found, low-level video
enumerations will also trigger a possibly cached audio enumeration
and the heuristic is run after both results are obtained.

This CL is preparation for full support for the groupId constrainable
property in getUserMedia() and related MediaStreamTrack methods.

Bug: 834281
Change-Id: I428c9a2cb65f755cdee366d6e43e4fe9ad898e20
Reviewed-on: https://chromium-review.googlesource.com/1019245
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Reviewed-by: Tommi <tommi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555697}