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/dtmfsender_unittest.cc b/webrtc/api/dtmfsender_unittest.cc
index e6fa7fc..efe568d 100644
--- a/webrtc/api/dtmfsender_unittest.cc
+++ b/webrtc/api/dtmfsender_unittest.cc
@@ -96,7 +96,7 @@
     return true;
   }
 
-  virtual sigslot::signal0<>* GetOnDestroyedSignal() {
+  sigslot::signal0<>* GetOnDestroyedSignal() override {
     return &SignalDestroyed;
   }