### 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_browser_window.cc
web_contents()->WasOccluded();
  BaseWindow::OnWindowHide();
}
gin_helper::WrappableBase* BrowserWindow::New(gin_helper::ErrorThrower thrower,
                                              gin::Arguments* args) {
  if (!Browser::Get()->is_ready()) {
    thrower.ThrowError("Cannot create BrowserWindow before app is ready");
    return nullptr;
  }
  if (args->Length() > 1) {
    args->ThrowError();
    return nullptr;
  }
  gin_helper::Dictionary options;
  if (!(args->Length() == 1 && args->GetNext(&options))) {
    options = gin::Dictionary::CreateEmpty(args->isolate());
  }
  return new BrowserWindow(args, options);
}
void BrowserWindow::BuildPrototype(v8::Isolate* isolate,
                                   v8::Local<v8::FunctionTemplate> prototype) {
  prototype->SetClassName(gin::StringToV8(isolate, "BrowserWindow"));
  gin_helper::ObjectTemplateBuilder(isolate, prototype->PrototypeTemplate())
      .SetMethod("focusOnWebView", &BrowserWindow::FocusOnWebView)
      .SetMethod("blurWebView", &BrowserWindow::BlurWebView)
      .SetMethod("isWebViewFocused", &BrowserWindow::IsWebViewFocused)
      .SetProperty("webContents", &BrowserWindow::GetWebContents);
}
v8::Local<v8::Value> BrowserWindow::From(v8::Isolate* isolate,