Surface CandidatePairChange event

In order to be able to detect and measure context around candidate pair changes.

Bug: webrtc:10419
Change-Id: Iab0d7e7c80d925d1aa44617fc35975fdc6bbc6b9
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/147340
Commit-Queue: Alex Drake <alexdrake@google.com>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Reviewed-by: Qingsi Wang <qingsi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#28779}

api/peer_connection_interface.h

@@ -1184,6 +1184,10 @@ class PeerConnectionObserver {
   // Called when the ICE connection receiving status changes.
   virtual void OnIceConnectionReceivingChange(bool receiving) {}
 
+  // Called when the selected candidate pair for the ICE connection changes.
+  virtual void OnIceSelectedCandidatePairChanged(
+      const cricket::CandidatePairChangeEvent& event) {}
+
   // This is called when a receiver and its track are created.
   // TODO(zhihuang): Make this pure virtual when all subclasses implement it.
   // Note: This is called with both Plan B and Unified Plan semantics. Unified