Make AudioBuffer::InterleaveTo const

The only non-const operation was a one-time initialization of a member only used in this function. I've moved it to the ctor.

BUG=webrtc:5298

Review-Url: https://codereview.webrtc.org/2741733002
Cr-Commit-Position: refs/heads/master@{#17223}
diff --git a/webrtc/modules/audio_processing/audio_buffer.h b/webrtc/modules/audio_processing/audio_buffer.h
index d0e79db..da75dbf 100644
--- a/webrtc/modules/audio_processing/audio_buffer.h
+++ b/webrtc/modules/audio_processing/audio_buffer.h
@@ -110,7 +110,7 @@
   void DeinterleaveFrom(AudioFrame* audioFrame);
   // If |data_changed| is false, only the non-audio data members will be copied
   // to |frame|.
-  void InterleaveTo(AudioFrame* frame, bool data_changed);
+  void InterleaveTo(AudioFrame* frame, bool data_changed) const;
 
   // Use for float deinterleaved data.
   void CopyFrom(const float* const* data, const StreamConfig& stream_config);