webrtc-internals: show track id, not track source id

BUG=chromium:894057

Change-Id: Ibeb60e70c08fccc45886476ed4f459cd09ae9b5b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2282921Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Philipp Hancke <philipp.hancke@googlemail.com>
Cr-Commit-Position: refs/heads/master@{#785323}

third_party/blink/renderer/modules/peerconnection/peer_connection_tracker.cc

@@ -167,7 +167,7 @@ String SerializeSender(const String& indent,
     result.Append("null");
   } else {
     result.Append("'");
-    result.Append(sender.Track()->Source()->Id());
+    result.Append(sender.Track()->Id());
     result.Append("'");
   }
   result.Append(",\n");
@@ -189,7 +189,7 @@ String SerializeReceiver(const String& indent,
   DCHECK(receiver.Track());
   result.Append(indent);
   result.Append("  track:'");
-  result.Append(receiver.Track()->Source()->Id());
+  result.Append(receiver.Track()->Id());
   result.Append("',\n");
   // streams:['id,'id'],
   result.Append(indent);