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/modules/audio_processing/audio_processing_impl.cc b/webrtc/modules/audio_processing/audio_processing_impl.cc
index 0027c66..dbc04b7 100644
--- a/webrtc/modules/audio_processing/audio_processing_impl.cc
+++ b/webrtc/modules/audio_processing/audio_processing_impl.cc
@@ -295,11 +295,11 @@
         formats_.rev_proc_format.num_channels(),
         rev_audio_buffer_out_num_frames));
     if (rev_conversion_needed()) {
-      render_.render_converter = rtc::ScopedToUnique(AudioConverter::Create(
+      render_.render_converter = AudioConverter::Create(
           formats_.api_format.reverse_input_stream().num_channels(),
           formats_.api_format.reverse_input_stream().num_frames(),
           formats_.api_format.reverse_output_stream().num_channels(),
-          formats_.api_format.reverse_output_stream().num_frames()));
+          formats_.api_format.reverse_output_stream().num_frames());
     } else {
       render_.render_converter.reset(nullptr);
     }