Fix all -Wnon-virtual-dtor 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 2594b6c10637996c3a554757a188da633d5b12cb..1386e8c2106cf75f0a807ffdd814135ec494b3f1 100644
--- a/webrtc/api/peerconnectioninterface_unittest.cc
+++ b/webrtc/api/peerconnectioninterface_unittest.cc
@@ -416,7 +416,7 @@ class MockTrackObserver : public ObserverInterface {
 class MockPeerConnectionObserver : public PeerConnectionObserver {
  public:
   MockPeerConnectionObserver() : remote_streams_(StreamCollection::Create()) {}
-  ~MockPeerConnectionObserver() {
+  virtual ~MockPeerConnectionObserver() {
   }
   void SetPeerConnectionInterface(PeerConnectionInterface* pc) {
     pc_ = pc;