Name change from stream label to stream id for spec compliance.

Bug: webrtc:7932
Change-Id: I66f33597342394083256f050cac2a00a68042302
Reviewed-on: https://webrtc-review.googlesource.com/59280
Commit-Queue: Seth Hampson <shampson@webrtc.org>
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22276}

api/rtptransceiverinterface.h

@@ -40,6 +40,8 @@ struct RtpTransceiverInit final {
   RtpTransceiverDirection direction = RtpTransceiverDirection::kSendRecv;
 
   // The added RtpTransceiver will be added to these streams.
+  // TODO(shampson): Change name to stream_id & update native wrapper's naming
+  // as well.
   // TODO(bugs.webrtc.org/7600): Not implemented.
   std::vector<std::string> stream_labels;