Use of override keyword to fix chromium trybot

TBR=tommi@webrtc.org, guidou@chromium.org

Review URL: https://codereview.webrtc.org/1302403007 .

Cr-Commit-Position: refs/heads/master@{#9890}
diff --git a/webrtc/p2p/base/dtlstransportchannel.h b/webrtc/p2p/base/dtlstransportchannel.h
index 68bf4c0..2090759 100644
--- a/webrtc/p2p/base/dtlstransportchannel.h
+++ b/webrtc/p2p/base/dtlstransportchannel.h
@@ -199,7 +199,7 @@
     channel_->OnCandidate(candidate);
   }
 
-  void SetReceivingTimeout(int receiving_timeout_ms) {
+  void SetReceivingTimeout(int receiving_timeout_ms) override {
     channel_->SetReceivingTimeout(receiving_timeout_ms);
   }
 
diff --git a/webrtc/p2p/base/transportchannelproxy.h b/webrtc/p2p/base/transportchannelproxy.h
index 3d55d10..e39398a 100644
--- a/webrtc/p2p/base/transportchannelproxy.h
+++ b/webrtc/p2p/base/transportchannelproxy.h
@@ -79,7 +79,7 @@
   void OnReadyToSend(TransportChannel* channel);
   void OnRouteChange(TransportChannel* channel, const Candidate& candidate);
 
-  void OnMessage(rtc::Message* message);
+  void OnMessage(rtc::Message* message) override;
 
   typedef std::pair<rtc::Socket::Option, int> OptionPair;
   typedef std::vector<OptionPair> OptionList;