WebRTC: Fix and enable -Woverloaded-virtual warnings.

webrtc/api/test/peerconnectiontestwrapper.h

Index: webrtc/api/test/peerconnectiontestwrapper.h
diff --git a/webrtc/api/test/peerconnectiontestwrapper.h b/webrtc/api/test/peerconnectiontestwrapper.h
index c56035de79678bb3107dd3f4ae307cc94945a7ac..3cdac492f1910fd297ee7673191f7282d526eb0a 100644
--- a/webrtc/api/test/peerconnectiontestwrapper.h
+++ b/webrtc/api/test/peerconnectiontestwrapper.h
@@ -24,6 +24,13 @@ class PeerConnectionTestWrapper
       public webrtc::CreateSessionDescriptionObserver,
       public sigslot::has_slots<> {
  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;
+
   static void Connect(PeerConnectionTestWrapper* caller,
                       PeerConnectionTestWrapper* callee);