chromeos: Check ALSA return values in mixer code.

chrome/browser/chromeos/audio_mixer_alsa.h

 // Copyright (c) 2010 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_CHROMEOS_AUDIO_MIXER_ALSA_H_
 #define CHROME_BROWSER_CHROMEOS_AUDIO_MIXER_ALSA_H_
 #pragma once
 
 #include "base/basictypes.h"
 #include "base/callback.h"
@@ skipping 59 matching lines @@
   // so actual_vol will contain the true volume that was set.  This information
   // can be used to further refine the volume by adjust a different mixer
   // element.  The rounding_bias is added in before rounding to the nearest
   // volume step (use 0.5 to round to nearest).
   bool SetElementVolume_Locked(_snd_mixer_elem* elem,
                                double new_vol,
                                double* actual_vol,
                                double rounding_bias);
 
   // In ALSA, the mixer element's 'switch' is turned off to mute.
+  // GetElementMuted_Locked() returns false on failure.
   bool GetElementMuted_Locked(_snd_mixer_elem* elem) const;
   void SetElementMuted_Locked(_snd_mixer_elem* elem, bool mute);
 
   // Volume range limits are computed once during InitializeAlsaMixer.
   double min_volume_;
   double max_volume_;
 
   // Muting is done by setting volume to minimum, so we must save the original.
   // This is the only state information kept in this object.  In some cases,
   // ALSA can report it has a volume switch and we can turn it off, but it has
@@ skipping 14 matching lines @@
   scoped_ptr<base::Thread> thread_;
 
   DISALLOW_COPY_AND_ASSIGN(AudioMixerAlsa);
 };
 
 }  // namespace chromeos
 
 DISABLE_RUNNABLE_METHOD_REFCOUNT(chromeos::AudioMixerAlsa);
 
 #endif  // CHROME_BROWSER_CHROMEOS_AUDIO_MIXER_ALSA_H_