Fix Chromium clang plugin warnings

NOTRY=true
BUG=webrtc:163

Review-Url: https://codereview.webrtc.org/2288153002
Cr-Commit-Position: refs/heads/master@{#13964}

webrtc/modules/audio_processing/echo_cancellation_impl.h

@@ -28,7 +28,7 @@ class EchoCancellationImpl : public EchoCancellation {
  public:
   EchoCancellationImpl(rtc::CriticalSection* crit_render,
                        rtc::CriticalSection* crit_capture);
-  virtual ~EchoCancellationImpl();
+  ~EchoCancellationImpl() override;
 
   int ProcessRenderAudio(const AudioBuffer* audio);
   int ProcessCaptureAudio(AudioBuffer* audio, int stream_delay_ms);