<webview>: Fix an issue with destroying an opener that has unattached guests.

chrome/renderer/resources/extensions/web_view_events.js

 // Copyright 2014 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.
 
 // Event management for WebViewInternal.
 
 var DeclarativeWebRequestSchema =
     requireNative('schema_registry').GetSchema('declarativeWebRequest');
 var EventBindings = require('event_bindings');
 var IdGenerator = requireNative('id_generator');
@@ skipping 465 matching lines @@
     return;
   }
 
   if (defaultPrevented) {
     // Make browser plugin track lifetime of |windowObj|.
     MessagingNatives.BindToGC(windowObj, function() {
       // Avoid showing a warning message if the decision has already been made.
       if (actionTaken) {
         return;
       }
+
+      var guestInstanceId = getGuestInstanceId();
+      if (!guestInstanceId) {
+        // If the opener is already gone, then we won't have its
+        // guestInstanceId.
+        return;
+      }
+
       WebView.setPermission(
-          getGuestInstanceId(), requestId, 'default', '', function(allowed) {
-        if (allowed) {
-          return;
-        }
-        showWarningMessage();
-      });
+          guestInstanceId, requestId, 'default', '', function(allowed) {
+            if (allowed) {
+              return;
+            }
+            showWarningMessage();
+          });
     });
   } else {
     actionTaken = true;
     // The default action is to discard the window.
     WebView.setPermission(
         getGuestInstanceId(), requestId, 'default', '', function(allowed) {
       if (allowed) {
         return;
       }
       showWarningMessage();
@@ skipping 96 matching lines @@
   }
 };
 
 WebViewEvents.prototype.handleSizeChangedEvent = function(
     event, webViewEvent) {
   this.webViewInternal.onSizeChanged(webViewEvent);
 };
 
 exports.WebViewEvents = WebViewEvents;
 exports.CreateEvent = CreateEvent;