alsa_output: Drain and pause pcm when stopping.

media/audio/linux/alsa_wrapper.cc

Index: media/audio/linux/alsa_wrapper.cc
diff --git a/media/audio/linux/alsa_wrapper.cc b/media/audio/linux/alsa_wrapper.cc
index 67331b4b17d407dcfacb4c6942ed54f77505105f..06110f8cb54626fc1276017efaa0192e073fe64d 100644
--- a/media/audio/linux/alsa_wrapper.cc
+++ b/media/audio/linux/alsa_wrapper.cc
@@ -43,6 +43,10 @@ int AlsaWrapper::PcmPrepare(snd_pcm_t* handle) {
   return snd_pcm_prepare(handle);
 }
 
+int AlsaWrapper::PcmDrain(snd_pcm_t* handle) {
+  return snd_pcm_drain(handle);
+}
+
 int AlsaWrapper::PcmDrop(snd_pcm_t* handle) {
   return snd_pcm_drop(handle);
 }