Add null check for MediaDevicesManager in MDDH destructor.

content/browser/renderer_host/media/media_stream_manager.cc

Index: content/browser/renderer_host/media/media_stream_manager.cc
diff --git a/content/browser/renderer_host/media/media_stream_manager.cc b/content/browser/renderer_host/media/media_stream_manager.cc
index 0da58a4dc1a807af796a83d476c08bf21425803a..029b0638c4e3ced5da47532ccb1c32e5b6504b51 100644
--- a/content/browser/renderer_host/media/media_stream_manager.cc
+++ b/content/browser/renderer_host/media/media_stream_manager.cc
@@ -457,7 +457,7 @@ AudioInputDeviceManager* MediaStreamManager::audio_input_device_manager() {
 
 MediaDevicesManager* MediaStreamManager::media_devices_manager() {
   DCHECK_CURRENTLY_ON(BrowserThread::IO);
-  DCHECK(media_devices_manager_.get());
+  // nullptr might be returned during shutdown.
   return media_devices_manager_.get();
 }