Turn webrtc::Vad into a pure virtual interface

webrtc/modules/audio_coding/codecs/cng/audio_encoder_cng.cc

Index: webrtc/modules/audio_coding/codecs/cng/audio_encoder_cng.cc
diff --git a/webrtc/modules/audio_coding/codecs/cng/audio_encoder_cng.cc b/webrtc/modules/audio_coding/codecs/cng/audio_encoder_cng.cc
index 171198b9a39eeee9beb2030d96dadabfd6e4b037..1ef22f65fafa253e5868b54c05999a09ca7a2928 100644
--- a/webrtc/modules/audio_coding/codecs/cng/audio_encoder_cng.cc
+++ b/webrtc/modules/audio_coding/codecs/cng/audio_encoder_cng.cc
@@ -54,7 +54,8 @@ AudioEncoderCng::AudioEncoderCng(const Config& config)
       num_cng_coefficients_(config.num_cng_coefficients),
       sid_frame_interval_ms_(config.sid_frame_interval_ms),
       last_frame_active_(true),
-      vad_(config.vad ? config.vad : new Vad(config.vad_mode)) {
+      vad_(config.vad ? rtc_make_scoped_ptr(config.vad)
+                      : CreateVad(config.vad_mode)) {
   CHECK(config.IsOk()) << "Invalid configuration.";
   cng_inst_ = CreateCngInst(SampleRateHz(), sid_frame_interval_ms_,
                             num_cng_coefficients_);