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/peerconnectionfactory.h b/webrtc/api/peerconnectionfactory.h
index 995c760..1992087 100644
--- a/webrtc/api/peerconnectionfactory.h
+++ b/webrtc/api/peerconnectionfactory.h
@@ -35,7 +35,7 @@
 
 class PeerConnectionFactory : public PeerConnectionFactoryInterface {
  public:
-  virtual void SetOptions(const Options& options) {
+  void SetOptions(const Options& options) override {
     options_ = options;
   }