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### Issue Details

* **Electron Version:** 5.0.0-beta.8
* **Operating System:** macOS 10.13.6
* **Last Known Working Electron version:**: 4.1.4

### Expected Behavior
<!-- A clear and concise description of what you expected to happen. -->

The file name should be shown in the save dialog, as it is in Electron 4:

<img width="798" src="https://user-images.githubusercontent.com/10314059/55932619-0f63fb00-5bf0-11e9-852e-7c9ecf22ed70.png">

### Actual Behavior
<!-- A clear and concise description of what actually happened. -->

No file name is shown:

<img width="797" src="https://user-images.githubusercontent.com/10314059/55932632-1985f980-5bf0-11e9-99f7-bb058a0a0134.png">

(Also, the dialog is attached to the window in Electron 5 but not 4, I'm not sure whether that's expected or not).

### To Reproduce

Create a link to a download in `index.html`:

```
<!DOCTYPE html>
<html>
  <head>
    <meta charset="UTF-8">
    <title>Hello World!</title>
  </head>
  <body>
    <a href="https://github.com/electron/electron/releases/download/v5.0.0-beta.8/electron-v5.0.0-beta.8-win32-x64.zip">download link</a>
  </body>
</html>
```
atom/browser/atom_download_manager_delegate.cc
}
  AtomBrowserContext* browser_context =
      static_cast<AtomBrowserContext*>(download_manager_->GetBrowserContext());
  base::FilePath default_download_path =
      browser_context->prefs()->GetFilePath(prefs::kDownloadDefaultDirectory);
  base::PostTaskWithTraitsAndReplyWithResult(
      FROM_HERE,
      {base::MayBlock(), base::TaskPriority::BEST_EFFORT,
       base::TaskShutdownBehavior::SKIP_ON_SHUTDOWN},
      base::BindOnce(&CreateDownloadPath, download->GetURL(),
                     download->GetContentDisposition(),
                     download->GetSuggestedFilename(), download->GetMimeType(),
                     default_download_path),
      base::BindOnce(&AtomDownloadManagerDelegate::OnDownloadPathGenerated,
                     weak_ptr_factory_.GetWeakPtr(), download->GetId(),
                     callback));
  return true;
}
bool AtomDownloadManagerDelegate::ShouldOpenDownload(
    download::DownloadItem* download,
    const content::DownloadOpenDelayedCallback& callback) {
  return true;
}
void AtomDownloadManagerDelegate::GetNextId(
    const content::DownloadIdCallback& callback) {
  static uint32_t next_id = download::DownloadItem::kInvalidId + 1;
  callback.Run(next_id++);
}
}