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-->

### Preflight Checklist
<!-- Please ensure you've completed the following steps by replacing [ ] with [x]-->

* [x] I have read the [Contributing Guidelines](https://github.com/electron/electron/blob/master/CONTRIBUTING.md) for this project.
* [x] I agree to follow the [Code of Conduct](https://github.com/electron/electron/blob/master/CODE_OF_CONDUCT.md) that this project adheres to.
* [x] I have searched the issue tracker for an issue that matches the one I want to file, without success.
(I have found similar issues on Windows: https://github.com/electron/electron/issues/2293 and https://github.com/electron/electron/pull/2556)

### Issue Details

* **Electron Version:**
  * <!-- (output of `node_modules/.bin/electron --version`) e.g. 4.0.3 --> 10.1.5
* **Operating System:**
  * <!-- (Platform and Version) e.g. macOS 10.13.6 / Windows 10 (1803) / Ubuntu 18.04 x64 --> Ubuntu 20.04.1 LTS x86_64
* **Last Known Working Electron version:**
  * <!-- (if applicable) e.g. 3.1.0 --> N/A

### Expected Behavior
<!-- A clear and concise description of what you expected to happen. -->
The message box created by `dialog.showMessageBox` should contain a close button, e.g. on macOS
<img width="798" alt="截屏2020-11-13 下午5 24 49" src="https://user-images.githubusercontent.com/16272760/99056813-d5ca5180-25d5-11eb-843b-987f40a54941.png">


### Actual Behavior
<!-- A clear and concise description of what actually happened. -->
Message box missing a close button on Ubuntu
![2020-11-13 17-33-01屏幕截图](https://user-images.githubusercontent.com/16272760/99057145-56894d80-25d6-11eb-855f-a72efff66bb7.png)


### To Reproduce
<!--
Your best chance of getting this bug looked at quickly is to provide an example.
-->

<!--
For bugs that can be encapsulated in a small experiment, you can use Electron Fiddle (https://github.com/electron/fiddle) to publish your example to a GitHub Gist and link it your bug report.
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If Fiddle is insufficient to produce an example, please provide an example REPOSITORY that can be cloned and run. You can fork electron-quick-start (https://github.com/electron/electron-quick-start) and include a link to the branch with your changes.
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<!--
If you provide a URL, please list the commands required to clone/setup/run your repo e.g.
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```sh
$ git clone https://github.com/stevenjoezhang/electron-quick-start -b master
$ npm install
$ npm start || electron .
```

### Screenshots
<!-- If applicable, add screenshots to help explain your problem. -->

### Additional Information
<!-- Add any other context about the problem here. -->
```
:~/electron-quick-start$ screenfetch
                          ./+o+-       
                  yyyyy- -yyyyyy+      OS: Ubuntu 20.04 focal
               ://+//////-yyyyyyo      Kernel: x86_64 Linux 5.4.0-53-generic
           .++ .:/++++++/-.+sss/`      Uptime: 1h 52m
         .:++o:  /++++++++/:--:/-      Packages: 2103
        o:+o+:++.`..```.-/oo+++++/     Shell: bash
       .:+o:+o/.          `+sssoo+/    Resolution: 3640x1920
  .++/+:+oo+o:`             /sssooo.   DE: GNOME 3.36.4
 /+++//+:`oo+o               /::--:.   WM: Mutter
 \+/+o+++`o++o               ++////.   WM Theme: Adwaita
  .++.o+++oo+:`             /dddhhh.   GTK Theme: Yaru [GTK2/3]
       .+.o+oo:.          `oddhhhh+    Icon Theme: ubuntu-mono-dark
        \+.++o+o``-````.:ohdhhhhh+     Font: Cantarell 11
         `:o+++ `ohhhhhhhhyo++os:      Disk: 58G / 1.2T (6%)
           .o:`.syhhhhhhh/.oo++o`      CPU: Intel Core i7-9700 @ 8x 4.7GHz [42.0°C]
               /osyyyyyyo++ooo+++/     GPU: Intel Corporation UHD Graphics 630 (Desktop 9 Series) (rev 02)
                   ````` +oo+++o\:     RAM: 6407MiB / 15794MiB
                          `oo++.      
```
shell/browser/ui/message_box_gtk.cc
switch (type) {
      case MessageBoxType::kInformation:
        return GTK_MESSAGE_INFO;
      case MessageBoxType::kWarning:
        return GTK_MESSAGE_WARNING;
      case MessageBoxType::kQuestion:
        return GTK_MESSAGE_QUESTION;
      case MessageBoxType::kError:
        return GTK_MESSAGE_ERROR;
      default:
        return GTK_MESSAGE_OTHER;
    }
  }
  const char* TranslateToStock(int id, const std::string& text) {
    const std::string lower = base::ToLowerASCII(text);
    if (lower == "cancel")
      return gtk_util::kCancelLabel;
    if (lower == "no")
      return gtk_util::kNoLabel;
    if (lower == "ok")
      return gtk_util::kOkLabel;
    if (lower == "yes")
      return gtk_util::kYesLabel;
    return text.c_str();
  }
  void Show() {