[webrtc-pc] new commits pushed by taylor-b

The following commits were just pushed by taylor-b to https://github.com/w3c/webrtc-pc:

* Adding note about the relationship between tracks of senders/receivers. (#1911)

* Adding note about the relationship between tracks of senders/receivers.

Fixes #1169.

Clarifies that there's not a 1:1 relationship between tracks sent by
one PeerConnection and received by the other (due to `replaceTrack`),
and the IDs are not guaranteed to match. So it's better to think of
a 1:1 relationship between senders on one side and receivers on the
other side, rather than tracks.
  by Taylor Brandstetter
https://github.com/w3c/webrtc-pc/commit/8938421cd34572c510fee3f8bfcd9808a95b7a68