Enable -Winconsistent-missing-override flag.

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 738b736dd8fd14d4f2c80317ea1de3618f2008d6..1a8dd57f08a6565f883c8ba5e73d69996fa1d623 100644
--- a/webrtc/api/peerconnectioninterface_unittest.cc
+++ b/webrtc/api/peerconnectioninterface_unittest.cc
@@ -424,8 +424,8 @@ class MockPeerConnectionObserver : public PeerConnectionObserver {
       state_ = pc_->signaling_state();
     }
   }
-  virtual void OnSignalingChange(
-      PeerConnectionInterface::SignalingState new_state) {
+  void OnSignalingChange(
+      PeerConnectionInterface::SignalingState new_state) override {
     EXPECT_EQ(pc_->signaling_state(), new_state);
     state_ = new_state;
   }