Implement GetAudioOutputDevices for remaining platforms, consolidate tests.

media/audio/linux/audio_manager_linux.cc

Index: media/audio/linux/audio_manager_linux.cc
diff --git a/media/audio/linux/audio_manager_linux.cc b/media/audio/linux/audio_manager_linux.cc
index 7596c2fe25e962c301b79abbb4e6a8190a94e286..708e4f26840bb51ef88873e760fe8a98df14b403 100644
--- a/media/audio/linux/audio_manager_linux.cc
+++ b/media/audio/linux/audio_manager_linux.cc
@@ -103,13 +103,13 @@ void AudioManagerLinux::ShowAudioInputSettings() {
 }
 
 void AudioManagerLinux::GetAudioInputDeviceNames(
-    media::AudioDeviceNames* device_names) {
+    AudioDeviceNames* device_names) {
   DCHECK(device_names->empty());
   GetAlsaAudioDevices(kStreamCapture, device_names);
 }
 
 void AudioManagerLinux::GetAudioOutputDeviceNames(
-    media::AudioDeviceNames* device_names) {
+    AudioDeviceNames* device_names) {
   DCHECK(device_names->empty());
   GetAlsaAudioDevices(kStreamPlayback, device_names);
 }