Add override declarations to PeerConnectionObserver subclasses, and delete obsolete methods.

BUG=None

Review-Url: https://codereview.webrtc.org/2660223002
Cr-Commit-Position: refs/heads/master@{#16374}
diff --git a/webrtc/examples/peerconnection/client/conductor.h b/webrtc/examples/peerconnection/client/conductor.h
index 74e5f6c..ba3e271 100644
--- a/webrtc/examples/peerconnection/client/conductor.h
+++ b/webrtc/examples/peerconnection/client/conductor.h
@@ -84,37 +84,37 @@
   // PeerConnectionClientObserver implementation.
   //
 
-  virtual void OnSignedIn();
+  void OnSignedIn() override;
 
-  virtual void OnDisconnected();
+  void OnDisconnected() override;
 
-  virtual void OnPeerConnected(int id, const std::string& name);
+  void OnPeerConnected(int id, const std::string& name) override;
 
-  virtual void OnPeerDisconnected(int id);
+  void OnPeerDisconnected(int id) override;
 
-  virtual void OnMessageFromPeer(int peer_id, const std::string& message);
+  void OnMessageFromPeer(int peer_id, const std::string& message) override;
 
-  virtual void OnMessageSent(int err);
+  void OnMessageSent(int err) override;
 
-  virtual void OnServerConnectionFailure();
+  void OnServerConnectionFailure() override;
 
   //
   // MainWndCallback implementation.
   //
 
-  virtual void StartLogin(const std::string& server, int port);
+  void StartLogin(const std::string& server, int port) override;
 
-  virtual void DisconnectFromServer();
+  void DisconnectFromServer() override;
 
-  virtual void ConnectToPeer(int peer_id);
+  void ConnectToPeer(int peer_id) override;
 
-  virtual void DisconnectFromCurrentPeer();
+  void DisconnectFromCurrentPeer() override;
 
-  virtual void UIThreadCallback(int msg_id, void* data);
+  void UIThreadCallback(int msg_id, void* data) override;
 
   // CreateSessionDescriptionObserver implementation.
-  virtual void OnSuccess(webrtc::SessionDescriptionInterface* desc);
-  virtual void OnFailure(const std::string& error);
+  void OnSuccess(webrtc::SessionDescriptionInterface* desc) override;
+  void OnFailure(const std::string& error) override;
 
  protected:
   // Send a message to the remote peer.