### 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
double factor) {
  if (factor < std::numeric_limits<double>::epsilon()) {
    thrower.ThrowError("'zoomFactor' must be a double greater than 0.0");
    return;
  }
  auto level = blink::PageZoomFactorToZoomLevel(factor);
  SetZoomLevel(level);
}
double WebContents::GetZoomFactor() const {
  auto level = GetZoomLevel();
  return blink::PageZoomLevelToZoomFactor(level);
}
void WebContents::SetTemporaryZoomLevel(double level) {
  zoom_controller_->SetTemporaryZoomLevel(level);
}
void WebContents::DoGetZoomLevel(
    electron::mojom::ElectronBrowser::DoGetZoomLevelCallback callback) {
  std::move(callback).Run(GetZoomLevel());
}
std::vector<base::FilePath> WebContents::GetPreloadPaths() const {
  auto result = SessionPreferences::GetValidPreloads(GetBrowserContext());
  if (auto* web_preferences = WebContentsPreferences::From(ontents())) {
    base::FilePath preload;
    if (web_preferences->GetPreloadPath(&preload)) {
      result.emplace_back(preload);
    }
  }
  return result;
}
v8::Local<v8::Value> WebContents::GetWebPreferences(