WebRTC: Fix and enable -Woverloaded-virtual warnings.

webrtc/api/peerconnectionfactory_unittest.cc

Index: webrtc/api/peerconnectionfactory_unittest.cc
diff --git a/webrtc/api/peerconnectionfactory_unittest.cc b/webrtc/api/peerconnectionfactory_unittest.cc
index 2d158e16af5b0cff9d323364e5f874a3e9fccbc8..9cd74ceb95fefbb7bdb7a95329269eadf2864189 100644
--- a/webrtc/api/peerconnectionfactory_unittest.cc
+++ b/webrtc/api/peerconnectionfactory_unittest.cc
@@ -63,6 +63,13 @@ static const char kTurnIceServerWithIPv6Address[] =
 
 class NullPeerConnectionObserver : public PeerConnectionObserver {
  public:
+  // We need these using declarations because there are two versions of each of
+  // the below methods and we only override one of them.
+  // TODO(deadbeef): Remove once there's only one version of the methods.
+  using PeerConnectionObserver::OnAddStream;
+  using PeerConnectionObserver::OnRemoveStream;
+  using PeerConnectionObserver::OnDataChannel;
+
   virtual ~NullPeerConnectionObserver() = default;
   virtual void OnMessage(const std::string& msg) {}
   virtual void OnSignalingMessage(const std::string& msg) {}