Replace scoped_ptr with unique_ptr in webrtc/common_audio/

(This is a re-land---without the real_fourier.h changes---of 11716, which was reverted in 11726.)

BUG=webrtc:5520

Review URL: https://codereview.webrtc.org/1731153002

Cr-Commit-Position: refs/heads/master@{#11742}
diff --git a/webrtc/common_audio/audio_converter.cc b/webrtc/common_audio/audio_converter.cc
index 9ebfabc..d6334b3 100644
--- a/webrtc/common_audio/audio_converter.cc
+++ b/webrtc/common_audio/audio_converter.cc
@@ -136,11 +136,11 @@
   ScopedVector<ChannelBuffer<float>> buffers_;
 };
 
-rtc::scoped_ptr<AudioConverter> AudioConverter::Create(size_t src_channels,
+std::unique_ptr<AudioConverter> AudioConverter::Create(size_t src_channels,
                                                        size_t src_frames,
                                                        size_t dst_channels,
                                                        size_t dst_frames) {
-  rtc::scoped_ptr<AudioConverter> sp;
+  std::unique_ptr<AudioConverter> sp;
   if (src_channels > dst_channels) {
     if (src_frames != dst_frames) {
       ScopedVector<AudioConverter> converters;