Enforce marking "override" for functions overriding Blink in components/test_runner/.

components/test_runner/mock_web_audio_device.h

Index: components/test_runner/mock_web_audio_device.h
diff --git a/components/test_runner/mock_web_audio_device.h b/components/test_runner/mock_web_audio_device.h
index 14ad9b1b2e6c69513944cfdde2046aa436cc61bf..1c45c4352965058776b50cdcf059a33f1f8fadc4 100644
--- a/components/test_runner/mock_web_audio_device.h
+++ b/components/test_runner/mock_web_audio_device.h
@@ -13,12 +13,12 @@ namespace test_runner {
 class MockWebAudioDevice : public blink::WebAudioDevice {
  public:
   explicit MockWebAudioDevice(double sample_rate);
-  virtual ~MockWebAudioDevice();
+  ~MockWebAudioDevice() override;
 
   // blink::WebAudioDevice:
-  virtual void start();
-  virtual void stop();
-  virtual double sampleRate();
+  void start() override;
+  void stop() override;
+  double sampleRate() override;
 
  private:
   double sample_rate_;