### Preflight Checklist
* [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.

### Issue Details

* **Electron Version:** 5.0.7
* **Operating System:** Windows 10

### Expected Behavior
When I navigate a `BrowserView` to a page that has `beforeunload` and `unload` handlers, I expect that closing the window will cause them to fire.

### Actual Behavior
They do not fire when closing the window.

Notably, if I don't use `BrowserView` and just navigate the `BrowserWindow` to the page, those handlers will execute.

### To Reproduce

`npm start` the code below (in main.js). In the devtools that opens up, run `window.onbeforeunload = () => true;`, then close the window. In the working case, the window will not close. In the broken case that uses `BrowserView`, it will.

```
const { app, BrowserWindow, BrowserView } = require('electron')

function brokenCase() {
	const mainWindow = new BrowserWindow();
	const view = new BrowserView();
	mainWindow.setBrowserView(view);
	view.webContents.loadURL("https://google.com");
	view.webContents.openDevTools();
}

function workingCase() {
	const mainWindow = new BrowserWindow();
	mainWindow.webContents.loadURL("https://google.com");
	mainWindow.webContents.openDevTools();
}

app.on('ready', brokenCase)
```

Maybe related to [this comment](https://github.com/electron/electron/issues/19194#issuecomment-544407723)
shell/browser/api/electron_api_web_contents.cc
std::string accept_languages =
      g_browser_process->GetApplicationLocale() + ",";
  for (auto const& language : electron::GetPreferredLanguages()) {
    if (language == g_browser_process->GetApplicationLocale())
      continue;
    accept_languages += language + ",";
  }
  accept_languages.pop_back();
  prefs->accept_languages = accept_languages;
#if defined(OS_LINUX) || defined(OS_WIN)
  
  static const base::NoDestructor<gfx::FontRenderParams> params(
      gfx::GetFontRenderParams(gfx::FontRenderParamsQuery(), nullptr));
  prefs->should_antialias_text = params->antialiasing;
  prefs->use_subpixel_positioning = params->subpixel_positioning;
  prefs->hinting = params->hinting;
  prefs->use_autohinter = params->autohinter;
  prefs->use_bitmaps = params->use_bitmaps;
  prefs->subpixel_rendering = params->subpixel_rendering;
#endif
  
  if (auto interval = GetCursorBlinkInterval())
    prefs->caret_blink_interval = *interval;
  
  
  
  if (!WebContentsPreferences::From(ontents())) {
    new WebContentsPreferences(ontents(), options);
  }