Add software audio mixing to the audio utils.

media/audio/audio_util.h

Index: media/audio/audio_util.h
===================================================================
--- media/audio/audio_util.h	(revision 125439)
+++ media/audio/audio_util.h	(working copy)
@@ -40,6 +40,14 @@
                                int bytes_per_sample,
                                float volume);
 
+// MixStreams() mixes 2 audio streams, adjusting volume on one of them.
+// Dst += Src * volume.

[vrk (LEFT CHROMIUM), 2012/03/12 18:40:24]

This function assumes |dst| and |src| have the same sample rate, right? Add
comment?

[enal1, 2012/03/12 21:20:53]

Done.

+MEDIA_EXPORT void MixStreams(void* dst,
+                             void* src,
+                             size_t buflen,
+                             int bytes_per_sample,
+                             float volume);
+
 // FoldChannels() does a software multichannel folding down to stereo.
 // Channel order is assumed to be 5.1 Dolby standard which is
 // front left, front right, center, surround left, surround right.