Add the last_sent_packet_id to the candidate pair change signal
so that the call knows which packet ids were sent on the previous candidate pair.
Note that packet_id is actually 16bits, so we can use -1 for values that are not set.

Also moved the tests for candidate pair changes to TestSelectConnectionBeforeNomination.

BUG=
R=deadbeef@webrtc.org, pthatcher@webrtc.org

Review URL: https://codereview.webrtc.org/1842093002 .

Cr-Commit-Position: refs/heads/master@{#12184}
diff --git a/webrtc/pc/channel.h b/webrtc/pc/channel.h
index e22196c..e05b6e5 100644
--- a/webrtc/pc/channel.h
+++ b/webrtc/pc/channel.h
@@ -209,7 +209,8 @@
 
   void OnSelectedCandidatePairChanged(
       TransportChannel* channel,
-      CandidatePairInterface* selected_candidate_pair);
+      CandidatePairInterface* selected_candidate_pair,
+      int last_sent_packet_id);
 
   bool PacketIsRtcp(const TransportChannel* channel, const char* data,
                     size_t len);