Adds default for PeerConnectionObserver::OnIceConnectionChange

It's planned to be deprecated so it should not be required.

Bug: webrtc:9883
Change-Id: I7daa922786d3cbf6bca38e205f4f57773f3f8448
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/147275
Commit-Queue: Sebastian Jansson <srte@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28715}
diff --git a/api/peer_connection_interface.h b/api/peer_connection_interface.h
index 8ffb8ae..b699609 100644
--- a/api/peer_connection_interface.h
+++ b/api/peer_connection_interface.h
@@ -1150,7 +1150,7 @@
   //
   // TODO(jonasolsson): deprecate and remove this.
   virtual void OnIceConnectionChange(
-      PeerConnectionInterface::IceConnectionState new_state) = 0;
+      PeerConnectionInterface::IceConnectionState new_state) {}
 
   // Called any time the standards-compliant IceConnectionState changes.
   virtual void OnStandardizedIceConnectionChange(