Fix constness of AudioBuffer accessors.

Don't return non-const pointers from const accessors and deal with the
spillover. Provide overloaded versions as needed.

Inspired by kwiberg:
https://webrtc-codereview.appspot.com/12379005/

R=bjornv@webrtc.org, kwiberg@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/15379004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6030 4adac7df-926f-26a2-2b94-8c16560cd09d
diff --git a/webrtc/common_audio/vad/webrtc_vad.c b/webrtc/common_audio/vad/webrtc_vad.c
index 3b31ef5..8a9b931 100644
--- a/webrtc/common_audio/vad/webrtc_vad.c
+++ b/webrtc/common_audio/vad/webrtc_vad.c
@@ -68,7 +68,7 @@
   return WebRtcVad_set_mode_core(self, mode);
 }
 
-int WebRtcVad_Process(VadInst* handle, int fs, int16_t* audio_frame,
+int WebRtcVad_Process(VadInst* handle, int fs, const int16_t* audio_frame,
                       int frame_length) {
   int vad = -1;
   VadInstT* self = (VadInstT*) handle;