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}

webrtc/base/callback_unittest.cc

@@ -34,7 +34,7 @@ class RefCountedBindTester : public RefCountInterface {
   int AddRef() const override {
     return ++count_;
   }
-  int Release() const {
+  int Release() const override {
     return --count_;
   }
   int RefCount() const { return count_; }