Don't leave renderer process frozen when canceling JavaScript dialogs.

chrome/browser/ui/app_modal_dialogs/javascript_app_modal_dialog.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/app_modal_dialogs/javascript_app_modal_dialog.h"
 
 #include "base/command_line.h"
 #include "chrome/browser/browser_shutdown.h"
 #include "chrome/browser/ui/app_modal_dialogs/native_app_modal_dialog.h"
 #include "chrome/common/chrome_switches.h"
@@ skipping 90 matching lines @@
 
 bool JavaScriptAppModalDialog::IsJavaScriptModalDialog() {
   return true;
 }
 
 void JavaScriptAppModalDialog::Invalidate() {
   if (!IsValid())
     return;
 
   AppModalDialog::Invalidate();
-  callback_.Reset();
+  if (!callback_.is_null()) {
+    callback_.Run(false, string16());
+    callback_.Reset();
+  }
   if (native_dialog())
     CloseModalDialog();
 }
 
 void JavaScriptAppModalDialog::OnCancel(bool suppress_js_messages) {
   // If we are shutting down and this is an onbeforeunload dialog, cancel the
   // shutdown.
   // TODO(sammc): Remove this when kEnableBatchedShutdown becomes mandatory.
   if (is_before_unload_dialog_ &&
       !CommandLine::ForCurrentProcess()->HasSwitch(
@@ skipping 32 matching lines @@
   override_prompt_text_ = override_prompt_text;
   use_override_prompt_text_ = true;
 }
 
 void JavaScriptAppModalDialog::NotifyDelegate(bool success,
                                               const string16& user_input,
                                               bool suppress_js_messages) {
   if (!IsValid())
     return;
 
-  callback_.Run(success, user_input);
+  if (!callback_.is_null()) {
+    callback_.Run(success, user_input);
+    callback_.Reset();
+  }
 
   // The callback_ above may delete web_contents_, thus removing the extra
   // data from the map owned by ChromeJavaScriptDialogManager. Make sure
   // to only use the data if still present. http://crbug.com/236476
   ExtraDataMap::iterator extra_data = extra_data_map_->find(web_contents());
   if (extra_data != extra_data_map_->end()) {
     extra_data->second.last_javascript_message_dismissal_ =
         base::TimeTicks::Now();
     extra_data->second.suppress_javascript_messages_ = suppress_js_messages;
   }
 
   // On Views, we can end up coming through this code path twice :(.
   // See crbug.com/63732.
   AppModalDialog::Invalidate();
 }