Add dummy default implementations for deprecated methods.

To aid deletion of deprecated methods in external child classes
of PeerConnectionFactoryInterface.

Bug: webrtc:9239
Change-Id: Idbc1c31285bceeb172a0b5bdb2b106f0c449dcb9
Reviewed-on: https://webrtc-review.googlesource.com/76623
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23229}
diff --git a/api/peerconnectioninterface.h b/api/peerconnectioninterface.h
index ed30c1d..7ec8353 100644
--- a/api/peerconnectioninterface.h
+++ b/api/peerconnectioninterface.h
@@ -1269,8 +1269,9 @@
       const PeerConnectionInterface::RTCConfiguration& configuration,
       std::unique_ptr<cricket::PortAllocator> allocator,
       std::unique_ptr<rtc::RTCCertificateGeneratorInterface> cert_generator,
-      PeerConnectionObserver* observer) = 0;
-
+      PeerConnectionObserver* observer) {
+    return nullptr;
+  }
   // Deprecated; should use RTCConfiguration for everything that previously
   // used constraints.
   virtual rtc::scoped_refptr<PeerConnectionInterface> CreatePeerConnection(
@@ -1278,7 +1279,9 @@
       const MediaConstraintsInterface* constraints,
       std::unique_ptr<cricket::PortAllocator> allocator,
       std::unique_ptr<rtc::RTCCertificateGeneratorInterface> cert_generator,
-      PeerConnectionObserver* observer) = 0;
+      PeerConnectionObserver* observer) {
+    return nullptr;
+  }
 
   virtual rtc::scoped_refptr<MediaStreamInterface> CreateLocalMediaStream(
       const std::string& stream_id) = 0;
@@ -1290,7 +1293,9 @@
   // Deprecated - use version above.
   // Can use CopyConstraintsIntoAudioOptions to bridge the gap.
   virtual rtc::scoped_refptr<AudioSourceInterface> CreateAudioSource(
-      const MediaConstraintsInterface* constraints) = 0;
+      const MediaConstraintsInterface* constraints) {
+    return nullptr;
+  }
 
   // Creates a VideoTrackSourceInterface from |capturer|.
   // TODO(deadbeef): We should aim to remove cricket::VideoCapturer from the