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

chrome/renderer/resources/extensions/web_view_events.js

Index: chrome/renderer/resources/extensions/web_view_events.js
diff --git a/chrome/renderer/resources/extensions/web_view_events.js b/chrome/renderer/resources/extensions/web_view_events.js
index 1a7a504bd7ba6479576fb7a465df79bfdc3ed3a9..4a32d93915098f231442d92cd6854917e3204ae1 100644
--- a/chrome/renderer/resources/extensions/web_view_events.js
+++ b/chrome/renderer/resources/extensions/web_view_events.js
@@ -483,13 +483,21 @@ WebViewEvents.prototype.handleNewWindowEvent = function(event, webViewEvent) {
       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;