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}

api/mediastreaminterface.cc

@@ -58,15 +58,4 @@ 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