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/webrtcsession.h b/webrtc/api/webrtcsession.h
index 89b77bb..970f967 100644
--- a/webrtc/api/webrtcsession.h
+++ b/webrtc/api/webrtcsession.h
@@ -267,10 +267,10 @@
                              const RtpParameters& parameters) override;
 
   // Implements DtmfProviderInterface.
-  virtual bool CanInsertDtmf(const std::string& track_id);
-  virtual bool InsertDtmf(const std::string& track_id,
-                          int code, int duration);
-  virtual sigslot::signal0<>* GetOnDestroyedSignal();
+  bool CanInsertDtmf(const std::string& track_id) override;
+  bool InsertDtmf(const std::string& track_id,
+                  int code, int duration) override;
+  sigslot::signal0<>* GetOnDestroyedSignal() override;
 
   // Implements DataChannelProviderInterface.
   bool SendData(const cricket::SendDataParams& params,