[ios] Fixed memory management bug in TestJavaScriptDialogPresenter.

ios/web/public/test/fakes/test_java_script_dialog_presenter.mm

 // Copyright 2017 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.
 
 #import "ios/web/public/test/fakes/test_java_script_dialog_presenter.h"
 
 namespace web {
 
 TestJavaScriptDialog::TestJavaScriptDialog() = default;
 
@@ skipping 10 matching lines @@
     WebState* web_state,
     const GURL& origin_url,
     JavaScriptDialogType java_script_dialog_type,
     NSString* message_text,
     NSString* default_prompt_text,
     const DialogClosedCallback& callback) {
   TestJavaScriptDialog dialog;
   dialog.web_state = web_state;
   dialog.origin_url = origin_url;
   dialog.java_script_dialog_type = java_script_dialog_type;
-  dialog.message_text.reset(message_text);
-  dialog.default_prompt_text.reset(default_prompt_text);
+  dialog.message_text.reset([message_text copy]);
+  dialog.default_prompt_text.reset([default_prompt_text copy]);
 
   requested_dialogs_.push_back(dialog);
 
   callback.Run(callback_success_argument_, callback_user_input_argument_.get());
 }
 
 void TestJavaScriptDialogPresenter::CancelDialogs(WebState* web_state) {
   cancel_dialogs_called_ = true;
 }
 
 }  // namespace web