Add null check for MediaDevicesManager in MDDH destructor.

content/browser/renderer_host/media/media_stream_manager.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/browser/renderer_host/media/media_stream_manager.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 439 matching lines @@
 }
 
 AudioInputDeviceManager* MediaStreamManager::audio_input_device_manager() {
   DCHECK_CURRENTLY_ON(BrowserThread::IO);
   DCHECK(audio_input_device_manager_.get());
   return audio_input_device_manager_.get();
 }
 
 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();
 }
 
 std::string MediaStreamManager::MakeMediaAccessRequest(
     int render_process_id,
     int render_frame_id,
     int page_request_id,
     const StreamControls& controls,
     const url::Origin& security_origin,
     const MediaRequestResponseCallback& callback) {
@@ skipping 1272 matching lines @@
   generate_stream_test_callback_ = test_callback;
 }
 
 #if defined(OS_WIN)
 void MediaStreamManager::FlushVideoCaptureThreadForTesting() {
   video_capture_thread_.FlushForTesting();
 }
 #endif
 
 }  // namespace content