WebUI: prevent WebContent to hold invalid pointer.

chrome/browser/ui/webui/constrained_web_dialog_ui.cc

 // Copyright (c) 2012 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 "chrome/browser/ui/webui/constrained_web_dialog_ui.h"
 
 #include <string>
 #include <vector>
 
 #include "base/bind.h"
@@ skipping 90 matching lines @@
 }
 
 // static
 void ConstrainedWebDialogUI::SetConstrainedDelegate(
     content::WebContents* web_contents,
     ConstrainedWebDialogDelegate* delegate) {
   web_contents->SetUserData(&kConstrainedWebDialogDelegateUserDataKey,
                             new ConstrainedWebDialogDelegateUserData(delegate));
 }
 
+// static
+void ConstrainedWebDialogUI::ClearConstrainedDelegate(
+    content::WebContents* web_contents) {
+  web_contents->RemoveUserData(&kConstrainedWebDialogDelegateUserDataKey);
+}
+
 ConstrainedWebDialogDelegate* ConstrainedWebDialogUI::GetConstrainedDelegate() {
   ConstrainedWebDialogDelegateUserData* user_data =
       static_cast<ConstrainedWebDialogDelegateUserData*>(
           web_ui()->GetWebContents()->

[Lei Zhang, 2017/04/06 09:12:54]

Another idea is for ConstrainedWebDialogUI to be a WebContentsObserver, and
override WebContentsDestroyed(). Then you'll know to return nullptr here. Though
getting ClearConstrainedDelegate() to work right feels cleaner.

               GetUserData(&kConstrainedWebDialogDelegateUserDataKey));
 
   return user_data ? user_data->delegate() : NULL;
 }