### 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
const std::string& file_system_id,
                                  const std::string& file_system_path,
                                  const std::string& type) {
  const GURL origin = ontents->GetURL().GetOrigin();
  std::string file_system_name =
      storage::GetIsolatedFileSystemName(origin, file_system_id);
  std::string root_url = storage::GetIsolatedFileSystemRootURIString(
      origin, file_system_id, kRootName);
  return FileSystem(type, file_system_name, root_url, file_system_path);
}
std::unique_ptr<base::DictionaryValue> CreateFileSystemValue(
    const FileSystem& file_system) {
  std::unique_ptr<base::DictionaryValue> file_system_value(
      new base::DictionaryValue());
  file_system_value->SetString("type", file_system.type);
  file_system_value->SetString("fileSystemName", file_system.file_system_name);
  file_system_value->SetString("rootURL", file_system.root_url);
  file_system_value->SetString("fileSystemPath", file_system.file_system_path);
  return file_system_value;
}
void WriteToFile(const base::FilePath& path, const std::string& content) {
  base::ScopedBlockingCall scoped_blocking_call(FROM_HERE,
                                                base::BlockingType::WILL_BLOCK);
  DCHECK(!path.empty());
  base::WriteFile(path, content.data(), content.size());
}
void AppendToFile(const base::FilePath& path, const std::string& content) {