WebRTC: Fix and enable -Woverloaded-virtual warnings.

webrtc/api/peerconnection_unittest.cc

Index: webrtc/api/peerconnection_unittest.cc
diff --git a/webrtc/api/peerconnection_unittest.cc b/webrtc/api/peerconnection_unittest.cc
index 422d37c2551d070df14072b234840a56d6a5ee29..3cfffe4631883f1997e4ca1bf26e4803f0f69b5b 100644
--- a/webrtc/api/peerconnection_unittest.cc
+++ b/webrtc/api/peerconnection_unittest.cc
@@ -188,6 +188,13 @@ class PeerConnectionTestClient : public webrtc::PeerConnectionObserver,
                                  public ObserverInterface,
                                  public rtc::MessageHandler {
  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;
+
   // If |config| is not provided, uses a default constructed RTCConfiguration.
   static PeerConnectionTestClient* CreateClientWithDtlsIdentityStore(
       const std::string& id,