Enable -Winconsistent-missing-override flag.

webrtc/base/callback_unittest.cc

 /*
  *  Copyright 2004 The WebRTC Project Authors. All rights reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 16 matching lines @@
   int a() { return 24; }
   int b(int x) const { return x * x; }
 };
 
 class RefCountedBindTester : public RefCountInterface {
  public:
   RefCountedBindTester() : count_(0) {}
   int AddRef() const override {
     return ++count_;
   }
-  int Release() const {
+  int Release() const override {
     return --count_;
   }
   int RefCount() const { return count_; }
 
  private:
   mutable int count_;
 };
 
 }  // namespace
 
@@ skipping 83 matching lines @@
     Callback0<void> cb = KeepRefUntilDone(t_scoped_ref);
     t_scoped_ref = nullptr;
     EXPECT_EQ(1, t.RefCount());
     cb();
     EXPECT_EQ(1, t.RefCount());
   }
   EXPECT_EQ(0, t.RefCount());
 }
 
 }  // namespace rtc