[chromecast] Slew stream volume changes in StreamMixerAlsa.

chromecast/media/cma/backend/alsa/stream_mixer_alsa.h

Index: chromecast/media/cma/backend/alsa/stream_mixer_alsa.h
diff --git a/chromecast/media/cma/backend/alsa/stream_mixer_alsa.h b/chromecast/media/cma/backend/alsa/stream_mixer_alsa.h
index e189d1db2749209a1511c4b2bea9bd56c351985e..1f5a2f21951879a86d8c5ded8336b3a56e06ba15 100644
--- a/chromecast/media/cma/backend/alsa/stream_mixer_alsa.h
+++ b/chromecast/media/cma/backend/alsa/stream_mixer_alsa.h
@@ -93,6 +93,12 @@ class StreamMixerAlsa {
     // MaxReadSize(), and |dest->frames()| shall be >= |frames|.
     virtual void GetResampledData(::media::AudioBus* dest, int frames) = 0;
 
+    // Scale |frames| frames at |src| by the current volume (smoothing as
+    // needed). Add the scaled result to |dest|.
+    virtual bool VolumeScaleAccumulate(const float* src,
+                                       int frames,
+                                       float* dest) = 0;
+
     // This is called for every InputQueue when the mixer writes data to ALSA
     // for any of its input streams.
     virtual void AfterWriteFrames(