Add ALSA support to volume keys

If PulseAudio is running, everything should behave as before, otherwise use ALSA API for adjusting volume.  The previous PulseAudioMixer was split into AudioMixerBase and audioMixerPusle, then AudioMixerAlsa was added.

BUG=chromium-os:10470
TEST=Volume keys should work even if pulseaudio disabled

Review URL: http://codereview.chromium.org/5859003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@71115 0039d316-1c4b-4281-b951-d872f2087c98
PulseAudioMixer::State PulseAudioMixer::CheckState() const {
AudioMixer::State AudioMixerPulse::GetState() const {
   AutoLock lock(mixer_state_lock_);
   if ((mixer_state_ == READY) &&
      (pa_context_get_state(pa_context_) != PA_CONTEXT_READY))
    mixer_state_ = IN_ERROR;
  return mixer_state_;
}

PulseAudioMixer::State PulseAudioMixer::CheckState() const {
   AutoLock lock(mixer_state_lock_);
   if ((mixer_state_ == READY) &&
      (pa_context_get_state(pa_context_) != PA_CONTEXT_READY))
    mixer_state_ = IN_ERROR;
  return mixer_state_;
}
