Add null check for MediaDevicesManager in MDDH destructor.

content/browser/renderer_host/media/media_devices_dispatcher_host.cc

Index: content/browser/renderer_host/media/media_devices_dispatcher_host.cc
diff --git a/content/browser/renderer_host/media/media_devices_dispatcher_host.cc b/content/browser/renderer_host/media/media_devices_dispatcher_host.cc
index d50e28a78e27a0f99215783ad6e34aad1ac3e0e1..c8e7b555c59225fc08c1b6476de5a9c208d847fa 100644
--- a/content/browser/renderer_host/media/media_devices_dispatcher_host.cc
+++ b/content/browser/renderer_host/media/media_devices_dispatcher_host.cc
@@ -95,6 +95,11 @@ MediaDevicesDispatcherHost::MediaDevicesDispatcherHost(
 
 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()