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/modules/audio_processing/gain_control_impl.cc b/webrtc/modules/audio_processing/gain_control_impl.cc
index e859044..a67b67e 100644
--- a/webrtc/modules/audio_processing/gain_control_impl.cc
+++ b/webrtc/modules/audio_processing/gain_control_impl.cc
@@ -59,7 +59,7 @@
 
   assert(audio->samples_per_split_channel() <= 160);
 
-  int16_t* mixed_data = audio->low_pass_split_data(0);
+  const int16_t* mixed_data = audio->low_pass_split_data(0);
   if (audio->num_channels() > 1) {
     audio->CopyAndMixLowPass(1);
     mixed_data = audio->mixed_low_pass_data(0);