Replace scoped_ptr with unique_ptr in webrtc/modules/audio_processing/

BUG=webrtc:5520

Review URL: https://codereview.webrtc.org/1710483002

Cr-Commit-Position: refs/heads/master@{#11684}
diff --git a/webrtc/modules/audio_processing/voice_detection_impl.cc b/webrtc/modules/audio_processing/voice_detection_impl.cc
index 22d218c..674a519 100644
--- a/webrtc/modules/audio_processing/voice_detection_impl.cc
+++ b/webrtc/modules/audio_processing/voice_detection_impl.cc
@@ -41,7 +41,7 @@
 void VoiceDetectionImpl::Initialize(int sample_rate_hz) {
   rtc::CritScope cs(crit_);
   sample_rate_hz_ = sample_rate_hz;
-  rtc::scoped_ptr<Vad> new_vad;
+  std::unique_ptr<Vad> new_vad;
   if (enabled_) {
     new_vad.reset(new Vad());
   }