Software volume control hook-up in linux.

media/audio/linux/alsa_output.h

Index: media/audio/linux/alsa_output.h
diff --git a/media/audio/linux/alsa_output.h b/media/audio/linux/alsa_output.h
index 850ab7f8d442fc1786f3b8a52b393040e1a2c075..2b619c0d1a684d92185664ae45f13cdcf4a53ff6 100644
--- a/media/audio/linux/alsa_output.h
+++ b/media/audio/linux/alsa_output.h
@@ -159,6 +159,9 @@ class AlsaPCMOutputStream :
   // Flag indicating that the resources are already cleaned.
   bool resources_released_;
 
+  // Volume level from 0 to 1.
+  float volume_;
+
   DISALLOW_COPY_AND_ASSIGN(AlsaPCMOutputStream);
 };