Mv camera enumeration to video capture service

Doc: go/mv-camera-enumeration

We define a new class VideoCaptureDevicesChangedObserver that
establishes a mojo connection between the video capture service and
the browser process and receives OnDeviceChanged events from
video_capture::DevicesChangedNotifier.

These changes are exposed under the feature
'kCameraMonitoringInVideoCaptureService' which will gradually be
rolled out following the finch process.

Bug: 1448798
Change-Id: I60048b3a2cc8f7b844bc823002f12419dde79f4e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5002121
Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Palak Agarwal <agpalak@chromium.org>
Reviewed-by: Mark Foltz <mfoltz@chromium.org>
Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1228892}