Modify the _vadActivity member of the AudioFrame passed to AudioProcessing.

This saves the user from having to explicitly check stream_has_voice(). It will allow typing detection to function, which relies on this behaviour.
Review URL: http://webrtc-codereview.appspot.com/144004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@621 4adac7df-926f-26a2-2b94-8c16560cd09d

src/modules/audio_processing/main/interface/audio_processing.h

@@ -486,6 +486,7 @@ class HighPassFilter {
 };
 
 // An estimation component used to retrieve level metrics.
+// NOTE: currently unavailable. All methods return errors.
 class LevelEstimator {
  public:
   virtual int Enable(bool enable) = 0;
@@ -539,6 +540,10 @@ class NoiseSuppression {
 // The voice activity detection (VAD) component analyzes the stream to
 // determine if voice is present. A facility is also provided to pass in an
 // external VAD decision.
+//
+// In addition to |stream_has_voice()| the VAD decision is provided through the
+// |AudioFrame| passed to |ProcessStream()|. The |_vadActivity| member will be
+// modified to reflect the current decision.
 class VoiceDetection {
  public:
   virtual int Enable(bool enable) = 0;