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This is a trivial change to move IMMNotificationClient implementation out of
AudioCapturerWin. Now the new DefaultAudioDeviceChangeDetector can unregister
itself from IMMDeviceEnumerator in destructor.

BUG=669070

Review-Url: https://codereview.chromium.org/2809293006
Cr-Commit-Position: refs/heads/master@{#466824}