### 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
static base::NoDestructor<base::IDMap<WebContents*>> s_all_ontents;
  return *s_all_ontents;
}
void OnCapturePageDone(gin_helper::Promise<gfx::Image> promise,
                       const SkBitmap& bitmap) {
  
  promise.Resolve(gfx::Image::CreateFrom1xBitmap(bitmap));
}
base::Optional<base::TimeDelta> GetCursorBlinkInterval() {
#if defined(OS_MAC)
  base::TimeDelta interval;
  if (ui::TextInsertionCaretBlinkPeriod(&interval))
    return interval;
#elif defined(OS_LINUX)
  if (auto* linux_ui = views::LinuxUI::instance())
    return linux_ui->GetCursorBlinkInterval();
#elif defined(OS_WIN)
  const auto system_msec = ::GetCaretBlinkTime();
  if (system_msec != 0) {
    return (system_msec == INFINITE)
               ? base::TimeDelta()
               : base::TimeDelta::FromMilliseconds(system_msec);
  }
#endif
  return base::nullopt;
}
#if BUILDFLAG(ENABLE_PRINTING)
bool IsDeviceNameValid(const std::u16string& device_name) {