[ios] Fixed memory management bug in TestJavaScriptDialogPresenter.

Copy strings before passing them to scoped_nsobjects.

BUG=679032

Review-Url: https://codereview.chromium.org/2612423003
Cr-Commit-Position: refs/heads/master@{#442093}

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

@@ -28,8 +28,8 @@ void TestJavaScriptDialogPresenter::RunJavaScriptDialog(
   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);