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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
#include "atom/browser/atom_download_manager_delegate.h"
#include <string>
#include <utility>
#include "atom/browser/api/atom_api_download_item.h"
#include "atom/browser/atom_browser_context.h"
#include "atom/browser/native_window.h"
#include "atom/browser/ui/file_dialog.h"
#include "atom/browser/web_contents_preferences.h"
#include "atom/common/native_mate_converters/callback.h"
#include "atom/common/options_switches.h"
#include "base/bind.h"
#include "base/files/file_util.h"
#include "base/task/post_task.h"
#include "chrome/common/pref_names.h"
#include "components/download/public/common/download_danger_type.h"
#include "components/prefs/pref_service.h"
#include "content/public/browser/browser_context.h"
#include "content/public/browser/browser_thread.h"
#include "content/public/browser/download_item_utils.h"
#include "content/public/browser/download_manager.h"
#include "net/base/filename_util.h"
namespace atom {
namespace {
base::FilePath CreateDownloadPath(const GURL& url,