Delete VoiceChannelTransport before deleting Channel in voe_cmd_test

Current voe_cmd_test shows following error when quitting:
DeRegisterExternalTransport() failed to locate channel.

This is to fix it.

BUG=
R=henrika@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/45349004

Cr-Commit-Position: refs/heads/master@{#9129}
1 file changed