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/base/callback_unittest.cc b/webrtc/base/callback_unittest.cc
index db294cd..aba1e0c 100644
--- a/webrtc/base/callback_unittest.cc
+++ b/webrtc/base/callback_unittest.cc
@@ -34,7 +34,7 @@
   int AddRef() const override {
     return ++count_;
   }
-  int Release() const {
+  int Release() const override {
     return --count_;
   }
   int RefCount() const { return count_; }