This removes the SessionStats object and replaces it with two
methods on PeerConnection: GetTransportNamesByMid and
GetTransportStatsByNames for use by the stats collectors. These
methods are more flexible and can cover cases where there are more
than one video/audio channel.
Bug: webrtc:8764
Change-Id: Id400cc548fc43675462ff6175a7fa9c9f4fd5948
Reviewed-on: https://webrtc-review.googlesource.com/47244
Commit-Queue: Steve Anton <steveanton@webrtc.org>
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21921}