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/rtpsender.h b/webrtc/api/rtpsender.h
index fe61cbd..86de765 100644
--- a/webrtc/api/rtpsender.h
+++ b/webrtc/api/rtpsender.h
@@ -98,8 +98,8 @@
 
   void Stop() override;
 
-  RtpParameters GetParameters() const;
-  bool SetParameters(const RtpParameters& parameters);
+  RtpParameters GetParameters() const override;
+  bool SetParameters(const RtpParameters& parameters) override;
 
  private:
   // TODO(nisse): Since SSRC == 0 is technically valid, figure out
@@ -164,8 +164,8 @@
 
   void Stop() override;
 
-  RtpParameters GetParameters() const;
-  bool SetParameters(const RtpParameters& parameters);
+  RtpParameters GetParameters() const override;
+  bool SetParameters(const RtpParameters& parameters) override;
 
  private:
   bool can_send_track() const { return track_ && ssrc_; }