Fixed crash by null-deref of XR::service_

This can happen when a VRService couldn't be started on browser process,
which calls an error handler callback that releases the service_ pointer.

Fixed it by adding a nullptr check around the usage of service_.

Also, checked if there are any other similar references.

Bug: 961971
Change-Id: Icda57d91280e46df07b88db858a0539b7fac1526
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1610467
Commit-Queue: Suman Kancherla <sumankancherla@chromium.org>
Reviewed-by: Klaus Weidner <klausw@chromium.org>
Reviewed-by: Bill Orr <billorr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659216}

third_party/blink/renderer/modules/xr/xr.cc

@@ -477,6 +477,9 @@ void XR::AddedEventListener(const AtomicString& event_type,
   EventTargetWithInlineData::AddedEventListener(event_type,
                                                 registered_listener);
 
+  if (!service_)
+    return;
+
   if (event_type == event_type_names::kDevicechange) {
     // Register for notifications if we haven't already.
     //