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This CL introduces MediaDevicesManager, which is now responsible for low-level
device enumerations, including using device monitoring to maintain updated
caches.

This CL also removes the enumeration functionality from the MediaStreamProvider
interface, as MediaDevicesManager is the new class resnposible for providing
enumeration results to be used to handle renderer-generated enumeration requests
and device-change notifications. Due to this, relatively minor changes are made
to AudioInputDeviceManager and VideoCaptureManager.

BUG=647660

Review-Url: https://codereview.chromium.org/2350693002
Cr-Commit-Position: refs/heads/master@{#420735}