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This fixes a bug where removing a device did not result in stopping all
its associated streams.

A unit test for this requires a minor refactoring to remove the
MediaStreamManager dependency from MediaDevicesManager, and will be
provided in a follow-up CL. This CL is deliberately simple in order to
make merging easier.

Bug: 856658
Change-Id: I84ef2856195cc7650bb08992d2095f8f853e9ea9
Reviewed-on: https://chromium-review.googlesource.com/1124323Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572176}