Fix null dereference in ExtensionsGuestViewManagerDelegate.

extensions/browser/guest_view/extensions_guest_view_manager_delegate.cc

 // Copyright 2015 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 "extensions/browser/guest_view/extensions_guest_view_manager_delegate.h"
 
 #include <utility>
 
 #include "components/guest_view/browser/guest_view_base.h"
 #include "components/guest_view/browser/guest_view_manager.h"
@@ skipping 39 matching lines @@
   // GetEventHistogramValue maps guest view event names to their histogram
   // value. It needs to be like this because the guest view component doesn't
   // know about extensions, so GuestViewEvent can't have an
   // extensions::events::HistogramValue as an argument.
   events::HistogramValue histogram_value =
       guest_view_events::GetEventHistogramValue(event_name);
   DCHECK_NE(events::UNKNOWN, histogram_value) << "Event " << event_name
                                               << " must have a histogram value";
 
   content::WebContents* owner = guest->owner_web_contents();
+  if (!owner)
+    return;  // Could happen at tab shutdown.
+
   EventRouter::DispatchEventToSender(owner, guest->browser_context(),
                                      guest->owner_host(), histogram_value,
                                      event_name, std::move(event_args),
                                      EventRouter::USER_GESTURE_UNKNOWN, info);
 }
 
 bool ExtensionsGuestViewManagerDelegate::IsGuestAvailableToContext(
     GuestViewBase* guest) {
   const Feature* feature =
       FeatureProvider::GetAPIFeature(guest->GetAPINamespace());
@@ skipping 26 matching lines @@
 void ExtensionsGuestViewManagerDelegate::RegisterAdditionalGuestViewTypes() {
   GuestViewManager* manager = GuestViewManager::FromBrowserContext(context_);
   manager->RegisterGuestViewType<AppViewGuest>();
   manager->RegisterGuestViewType<ExtensionOptionsGuest>();
   manager->RegisterGuestViewType<ExtensionViewGuest>();
   manager->RegisterGuestViewType<MimeHandlerViewGuest>();
   manager->RegisterGuestViewType<WebViewGuest>();
 }
 
 }  // namespace extensions