This removes code from DataChannelController that exposes an internal vector of data channels and puts the onus of returning stats for a data channel, on the data channel object itself. This will come in handy as we make threading changes to the data channel object. Change-Id: Ie164cc5823cd5f9782fc5c9a63aa4c76b8229639 Bug: webrtc:11547, webrtc:11687 Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/177244 Commit-Queue: Tommi <tommi@webrtc.org> Reviewed-by: Harald Alvestrand <hta@webrtc.org> Cr-Commit-Position: refs/heads/master@{#31533}