Mark unused/deprecated DTMF methods for removal

Bug: webrtc:9426
Change-Id: Ic0937d9d388092376cbaa2084e38de953a25e2dc
Reviewed-on: https://webrtc-review.googlesource.com/84246
Reviewed-by: Seth Hampson <shampson@webrtc.org>
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Commit-Queue: Steve Anton <steveanton@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23671}

api/dtmfsenderinterface.h

@@ -74,7 +74,8 @@ class DtmfSenderInterface : public rtc::RefCountInterface {
   // Returns the track given as argument to the constructor. Only exists for
   // backwards compatibilty; now that DtmfSenders are tied to RtpSenders, it's
   // no longer relevant.
-  virtual const AudioTrackInterface* track() const = 0;
+  // TODO(bugs.webrtc.org/9426): Remove this method.
+  virtual const AudioTrackInterface* track() const { return nullptr; }
 
   // Returns the tones remaining to be played out.
   virtual std::string tones() const = 0;

api/peerconnectioninterface.h

@@ -745,8 +745,11 @@ class PeerConnectionInterface : public rtc::RefCountInterface {
   // This API is no longer part of the standard; instead DtmfSenders are
   // obtained from RtpSenders. Which is what the implementation does; it finds
   // an RtpSender for |track| and just returns its DtmfSender.
+  // TODO(bugs.webrtc.org/9426): Remove this method.
   virtual rtc::scoped_refptr<DtmfSenderInterface> CreateDtmfSender(
-      AudioTrackInterface* track) = 0;
+      AudioTrackInterface* track) {
+    return nullptr;
+  }
 
   // TODO(deadbeef): Make these pure virtual once all subclasses implement them.