Provide a default no-op implementation of RegisterUMAObserver in
PeerConnectionInterface.

This allows the implementations of PeerConnectionInterface to deprecate
this method.

Bug: None
Change-Id: I54b56206ebac2486f112e09137c9def225683297
Reviewed-on: https://webrtc-review.googlesource.com/89261
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24011}
diff --git a/api/peerconnectioninterface.h b/api/peerconnectioninterface.h
index c99bc9f..90a99a6 100644
--- a/api/peerconnectioninterface.h
+++ b/api/peerconnectioninterface.h
@@ -960,7 +960,10 @@
   // this method takes a reference. RegisterUMAObserver(nullptr) will release
   // the reference.
   // TODO(deadbeef): Take argument as scoped_refptr?
-  virtual void RegisterUMAObserver(UMAObserver* observer) = 0;
+  //
+  // This method is soon to be deprecated. This no-op default implementation
+  // allows the implementations of the interface to remove this method.
+  virtual void RegisterUMAObserver(UMAObserver* observer) {}
 
   // 0 <= min <= current <= max should hold for set parameters.
   struct BitrateParameters {