alsa_output: Drain and pause pcm when stopping.

media/audio/linux/alsa_output.cc

Index: media/audio/linux/alsa_output.cc
diff --git a/media/audio/linux/alsa_output.cc b/media/audio/linux/alsa_output.cc
index ce301cef903713b1629b85012064fc3802969d83..d8653a8475cf2ebb0e2a6f82e0b73f4154818039 100644
--- a/media/audio/linux/alsa_output.cc
+++ b/media/audio/linux/alsa_output.cc
@@ -335,6 +335,9 @@ void AlsaPcmOutputStream::Stop() {
   // Reset the callback, so that it is not called anymore.
   set_source_callback(NULL);
 
+  // Finish playing samples and pause the pcm.
+  wrapper_->PcmDrain(playback_handle_);
+
   TransitionTo(kIsStopped);
 }