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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
uint32_t download_id,
    const content::DownloadTargetCallback& callback,
    const base::FilePath& default_path) {
  DCHECK_CURRENTLY_ON(content::BrowserThread::UI);
  auto* item = download_manager_->GetDownload(download_id);
  if (!item)
    return;
  NativeWindow* window = nullptr;
  content::WebContents* web_contents =
      content::DownloadItemUtils::GetWebContents(item);
  auto* relay =
      web_contents ? NativeWindowRelay::FromWebContents(web_contents) : nullptr;
  if (relay)
    window = relay->GetNativeWindow();
  
  base::FilePath path;
  GetItemSavePath(item, &path);
  if (path.empty()) {
    file_dialog::DialogSettings settings;
    GetItemSaveDialogOptions(item, &settings);
    if (!settings.parent_window)
      settings.parent_window = window;
    if (settings.title.size() == 0)
      settings.title = item->GetURL().spec();
    if (!settings.default_path.empty())
      settings.default_path = default_path;
    auto* web_preferences = WebContentsPreferences::From(web_contents);
    const bool offscreen =
        !web_preferences || web_preferences->IsEnabled(options::kOffscreen);