### 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:** 11.2.1
* **Operating System:** Windows 10

### Expected Behavior
I expect that `will-prevent-unload` will fire when closing a BrowserView with a page that cancels the `beforeunload` event

### Actual Behavior
`will-prevent-unload` never fires. It only fires for BrowserWindows.

### To Reproduce
1. start main.js
2. in the devtools, execute: `onbeforeunload = (e) => { e.preventDefault(); e.returnValue = false; }`
3. Close the window.
4. The window does close, but the `will-prevent-unload` handler never executes

```
const { app, BrowserWindow, BrowserView } = require('electron')

async function createWindow () {
  const win = new BrowserWindow();
  const view = new BrowserView();

  view.setBounds({x: 0, y: 0, width: 600, height: 600});
  win.setBrowserView(view);

  await view.webContents.loadURL('https://google.com')
  view.webContents.openDevTools();

  view.webContents.on("will-prevent-unload", (e) => {
    e.preventDefault();
  });
}

app.whenReady().then(createWindow)

app.on('window-all-closed', () => {
  if (process.platform !== 'darwin') {
    app.quit()
  }
});
```

Related (if not a dupe) of [this issue](https://github.com/electron/electron/issues/19194)
shell/browser/api/electron_api_web_contents.cc
auto file_system_paths = GetAddedFileSystemPaths(GetDevToolsWebContents());
  if (file_system_paths.empty()) {
    base::ListValue empty_file_system_value;
    inspectable_ontents_->CallClientFunction(
        "DevToolsAPI.fileSystemsLoaded", &empty_file_system_value, nullptr,
        nullptr);
    return;
  }
  std::vector<FileSystem> file_systems;
  for (const auto& file_system_path : file_system_paths) {
    base::FilePath path =
        base::FilePath::FromUTF8Unsafe(file_system_path.first);
    std::string file_system_id =
        RegisterFileSystem(GetDevToolsWebContents(), path);
    FileSystem file_system =
        CreateFileSystemStruct(GetDevToolsWebContents(), file_system_id,
                               file_system_path.first, file_system_path.second);
    file_systems.push_back(file_system);
  }
  base::ListValue file_system_value;
  for (const auto& file_system : file_systems)
    file_system_value.Append(CreateFileSystemValue(file_system));
  inspectable_ontents_->CallClientFunction(
      "DevToolsAPI.fileSystemsLoaded", &file_system_value, nullptr, nullptr);
}
void WebContents::DevToolsAddFileSystem(