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/audio_buffer.h b/webrtc/modules/audio_processing/audio_buffer.h
index eaf53eb..79f4689 100644
--- a/webrtc/modules/audio_processing/audio_buffer.h
+++ b/webrtc/modules/audio_processing/audio_buffer.h
@@ -55,15 +55,18 @@
   int samples_per_split_channel() const;
   int samples_per_keyboard_channel() const;
 
-  int16_t* data(int channel) const;
-  int16_t* low_pass_split_data(int channel) const;
-  int16_t* high_pass_split_data(int channel) const;
-  int16_t* mixed_data(int channel) const;
-  int16_t* mixed_low_pass_data(int channel) const;
-  int16_t* low_pass_reference(int channel) const;
+  int16_t* data(int channel);
+  const int16_t* data(int channel) const;
+  int16_t* low_pass_split_data(int channel);
+  const int16_t* low_pass_split_data(int channel) const;
+  int16_t* high_pass_split_data(int channel);
+  const int16_t* high_pass_split_data(int channel) const;
+  const int16_t* mixed_data(int channel) const;
+  const int16_t* mixed_low_pass_data(int channel) const;
+  const int16_t* low_pass_reference(int channel) const;
   const float* keyboard_data() const;
 
-  SplitFilterStates* filter_states(int channel) const;
+  SplitFilterStates* filter_states(int channel);
 
   void set_activity(AudioFrame::VADActivity activity);
   AudioFrame::VADActivity activity() const;