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/mediastreaminterface.cc

@@ -58,4 +58,15 @@ AudioTrackInterface::GetAudioProcessor() {
   return nullptr;
 }
 
+// TODO(shampson): Remove this once downstreams are using id().
+std::string MediaStreamInterface::label() const {
+  return id();
+}
+
+// TODO(shampson): Remove this default implementation once downstreams have
+// implemented.
+std::string MediaStreamInterface::id() const {
+  return std::string();
+}
+
 }  // namespace webrtc