Reorder AudioMessageFilter shutdown. Remove spammy DCHECK.

content/renderer/media/audio_renderer_mixer_manager.cc

Index: content/renderer/media/audio_renderer_mixer_manager.cc
diff --git a/content/renderer/media/audio_renderer_mixer_manager.cc b/content/renderer/media/audio_renderer_mixer_manager.cc
index b2d743e0b96e6dd240ea4a7fa15c77b2ae3a049b..901807bad19e865da43a8d860cbb43a56ac0c015 100644
--- a/content/renderer/media/audio_renderer_mixer_manager.cc
+++ b/content/renderer/media/audio_renderer_mixer_manager.cc
@@ -21,7 +21,9 @@ AudioRendererMixerManager::AudioRendererMixerManager(
 }
 
 AudioRendererMixerManager::~AudioRendererMixerManager() {
-  DCHECK(mixers_.empty());
+  // References to AudioRendererMixers may be owned by garbage collected
+  // objects.  During process shutdown they may be leaked, so, transitively,
+  // |mixers_| may leak (i.e., may be non-empty at this time) as well.
 }
 
 media::AudioRendererMixerInput* AudioRendererMixerManager::CreateInput(