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AVFoundation calls made by the video-capture service on its main thread
require a CFRunLoop which is provided by a UI message loop.
This fixes a number of regressions related to detection of plugged/unplugged
devices that appeared when the default message loops on Mac were changed from
CFRunLoop to Default.

Bug: 834581
Change-Id: If565f400e52407c2f71e32f93e9472556051bad1
Reviewed-on: https://chromium-review.googlesource.com/1049931Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557202}