WebRTC: Fix and enable -Woverloaded-virtual warnings.

webrtc/api/peerconnectioninterface_unittest.cc

Index: webrtc/api/peerconnectioninterface_unittest.cc
diff --git a/webrtc/api/peerconnectioninterface_unittest.cc b/webrtc/api/peerconnectioninterface_unittest.cc
index a4a11e1252e5720d5cc71386a43bb125c7f5cb7d..f38a1d35aa5e08391b5d355f28cb3c912ff52594 100644
--- a/webrtc/api/peerconnectioninterface_unittest.cc
+++ b/webrtc/api/peerconnectioninterface_unittest.cc
@@ -435,6 +435,13 @@ class MockTrackObserver : public ObserverInterface {
 
 class MockPeerConnectionObserver : 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;
+
   MockPeerConnectionObserver() : remote_streams_(StreamCollection::Create()) {}
   virtual ~MockPeerConnectionObserver() {
   }