Fix data races during VideoAdapterTest tear-down.
Fix data races during VideoAdapterTest tear-down.

Explicitly disconnect the VideoCapturer to avoid frames being
delivered during listener destruction. This manifested only on DrMemory
Full on Windows which I was able to repro locally.

BUG=3671
R=henrike@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/15289004

Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 75c3ec17636319b77be24459efa529d14e76d7f1

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