### 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
int bottom = 0;
      margins.Get("bottom", &bottom);
      custom_margins.SetIntKey(printing::kSettingMarginBottom, bottom);
      int left = 0;
      margins.Get("left", &left);
      custom_margins.SetIntKey(printing::kSettingMarginLeft, left);
      int right = 0;
      margins.Get("right", &right);
      custom_margins.SetIntKey(printing::kSettingMarginRight, right);
      settings.SetPath(printing::kSettingMarginsCustom,
                       std::move(custom_margins));
    }
  } else {
    settings.SetIntKey(
        printing::kSettingMarginsType,
        static_cast<int>(printing::mojom::MarginType::kDefaultMargins));
  }
   Set whether to print color or greyscale
  bool print_color = true;
  options.Get("color", &print_color);
  auto const color_model = print_color ? printing::mojom::ColorModel::kColor
                                       : printing::mojom::ColorModel::kGray;
  settings.SetIntKey(printing::kSettingColor, static_cast<int>(color_model));
   Is the orientation landscape or portrait.
  bool landscape = false;
  options.Get("landscape", &landscape);
  settings.SetBoolKey(printing::kSettingLandscape, landscape);
   We set the default to the system's default printer and only update
   if at the Chromium level if the user overrides.
   Printer device name as opened by the OS.