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/java/jni/peerconnection_jni.cc b/webrtc/api/java/jni/peerconnection_jni.cc
index 34358ad..10b8134 100644
--- a/webrtc/api/java/jni/peerconnection_jni.cc
+++ b/webrtc/api/java/jni/peerconnection_jni.cc
@@ -202,7 +202,7 @@
   }
 
   void OnIceCandidatesRemoved(
-      const std::vector<cricket::Candidate>& candidates) {
+      const std::vector<cricket::Candidate>& candidates) override {
     ScopedLocalRefFrame local_ref_frame(jni());
     jobjectArray candidates_array = ToJavaCandidateArray(jni(), candidates);
     jmethodID m =