Add null check for MediaDevicesManager in MDDH destructor.

content/browser/renderer_host/media/media_devices_dispatcher_host.cc

 // Copyright 2016 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_devices_dispatcher_host.h"
 
 #include <stddef.h>
 
 #include <utility>
 #include <vector>
@@ skipping 77 matching lines @@
       device_id_salt_(device_id_salt),
       group_id_salt_(ResourceContext::CreateRandomMediaDeviceIDSalt()),
       media_stream_manager_(media_stream_manager),
       permission_checker_(base::MakeUnique<MediaDevicesPermissionChecker>()),
       weak_factory_(this) {
   DCHECK_CURRENTLY_ON(BrowserThread::IO);
 }
 
 MediaDevicesDispatcherHost::~MediaDevicesDispatcherHost() {
   DCHECK_CURRENTLY_ON(BrowserThread::IO);
+  // It may happen that media_devices_manager() is destroyed before MDDH on some
+  // shutdown scenarios.
+  if (!media_stream_manager_->media_devices_manager())
+    return;
+
   for (size_t i = 0; i < NUM_MEDIA_DEVICE_TYPES; ++i) {
     if (!device_change_subscriptions_[i].empty()) {
       media_stream_manager_->media_devices_manager()
           ->UnsubscribeDeviceChangeNotifications(
               static_cast<MediaDeviceType>(i), this);
     }
   }
 }
 
 void MediaDevicesDispatcherHost::EnumerateDevices(
@@ skipping 179 matching lines @@
     for (const auto& device_info : enumeration[i]) {
       result[i].push_back(TranslateDeviceInfo(has_permissions[i],
                                               device_id_salt_, group_id_salt_,
                                               security_origin, device_info));
     }
   }
   client_callback.Run(result);
 }
 
 }  // namespace content