Pass it to MediaController constructor and down to WebRtcVideoEngine2 and WebRtcVoiceEngine. Follows discussion on https://codereview.webrtc.org/1646253004/ TBR=pthatcher@webrtc.org BUG=webrtc:5438 Review URL: https://codereview.webrtc.org/1670153003 Cr-Commit-Position: refs/heads/master@{#11595}