Detach audio thread checker when setting a new audio renderer in WebRtcAudioDeviceImpl.
Detach audio thread checker when setting a new audio renderer in WebRtcAudioDeviceImpl.

BUG=543991

Review URL: https://codereview.chromium.org/1407423002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#354680}

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