Final name changing of MediaStreamInterface.label() to id().

Downstreams have been updated, and this now updates all uses of label()
to id() within WebRTC code. This change also makes id() pure virtual and
removes label().

Bug: webrtc:8977
Change-Id: Ib045ea4fabba6f14447c64875c7aeba87dc2be24
Reviewed-on: https://webrtc-review.googlesource.com/60382
Reviewed-by: Per Kjellander <perkj@webrtc.org>
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Commit-Queue: Seth Hampson <shampson@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22431}
diff --git a/examples/unityplugin/simple_peer_connection.cc b/examples/unityplugin/simple_peer_connection.cc
index a830f91..2b8ef93 100644
--- a/examples/unityplugin/simple_peer_connection.cc
+++ b/examples/unityplugin/simple_peer_connection.cc
@@ -378,7 +378,7 @@
 
 void SimplePeerConnection::OnAddStream(
     rtc::scoped_refptr<webrtc::MediaStreamInterface> stream) {
-  RTC_LOG(INFO) << __FUNCTION__ << " " << stream->label();
+  RTC_LOG(INFO) << __FUNCTION__ << " " << stream->id();
   remote_stream_ = stream;
   if (remote_video_observer_ && !remote_stream_->GetVideoTracks().empty()) {
     remote_stream_->GetVideoTracks()[0]->AddOrUpdateSink(
@@ -491,7 +491,7 @@
   typedef std::pair<std::string,
                     rtc::scoped_refptr<webrtc::MediaStreamInterface>>
       MediaStreamPair;
-  active_streams_.insert(MediaStreamPair(stream->label(), stream));
+  active_streams_.insert(MediaStreamPair(stream->id(), stream));
 }
 
 bool SimplePeerConnection::CreateDataChannel() {