Stop video candidates getting down to audio.

Second attempt at adding a check to make sure that the video
transportproxy doesn't send down candidates to the audio transport
channel when things are bundled.

BUG=4665
R=juberti@google.com, pthatcher@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/50059004

Cr-Commit-Position: refs/heads/master@{#9316}
2 files changed