This changes PeerConnectionObserver to not default the stream
events as pure virtual. Applications which have switched to using
OnAddTrack and OnRemoveTrack will no longer need to implement
these callbacks.
Bug: webrtc:8587
Change-Id: I659ce7b5a208ebfcb29e899dd17916ae0072d3cc
Reviewed-on: https://webrtc-review.googlesource.com/39384
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Reviewed-by: Peter Thatcher <pthatcher@webrtc.org>
Commit-Queue: Taylor Brandstetter <deadbeef@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#21665}