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/modules/video_coding/jitter_buffer_unittest.cc

@@ -215,7 +215,7 @@ class TestBasicJitterBuffer : public ::testing::TestWithParam<std::string>,
 
  protected:
   TestBasicJitterBuffer() : scoped_field_trial_(GetParam()) {}
-  virtual void SetUp() {
+  void SetUp() override {
     clock_.reset(new SimulatedClock(0));
     jitter_buffer_.reset(new VCMJitterBuffer(
         clock_.get(),