Enable -Winconsistent-missing-override flag.

The problem with gmock is worked around by commenting out any other override declarations in classes using gmock.

NOPRESUBMIT=True
BUG=webrtc:3970

Review-Url: https://codereview.webrtc.org/1921653002
Cr-Commit-Position: refs/heads/master@{#12563}
diff --git a/webrtc/api/peerconnectioninterface_unittest.cc b/webrtc/api/peerconnectioninterface_unittest.cc
index 738b736..1a8dd57 100644
--- a/webrtc/api/peerconnectioninterface_unittest.cc
+++ b/webrtc/api/peerconnectioninterface_unittest.cc
@@ -424,8 +424,8 @@
       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;
   }