Fix all -Wnon-virtual-dtor warnings.

This is needed to get the GN build going for several parts
of the code tree.

BUG=webrtc:3307
NOTRY=True
R=henrika@webrtc.org, nisse@webrtc.org

Review URL: https://codereview.webrtc.org/1928653005 .

Cr-Commit-Position: refs/heads/master@{#12693}

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;