Replace scoped_ptr with unique_ptr in webrtc/modules/audio_processing/vad/

BUG=webrtc:5520

Review URL: https://codereview.webrtc.org/1699003002

Cr-Commit-Position: refs/heads/master@{#11657}
diff --git a/webrtc/modules/audio_processing/vad/common.h b/webrtc/modules/audio_processing/vad/common.h
index be99c1c..4b7fb50 100644
--- a/webrtc/modules/audio_processing/vad/common.h
+++ b/webrtc/modules/audio_processing/vad/common.h
@@ -11,6 +11,8 @@
 #ifndef WEBRTC_MODULES_AUDIO_PROCESSING_VAD_COMMON_H_
 #define WEBRTC_MODULES_AUDIO_PROCESSING_VAD_COMMON_H_
 
+#include <stddef.h>
+
 static const int kSampleRateHz = 16000;
 static const size_t kLength10Ms = kSampleRateHz / 100;
 static const size_t kMaxNumFrames = 4;