### 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
content::RenderFrameHost* frame_host =
      navigation_handle->GetRenderFrameHost();
  int frame_process_id = -1, frame_routing_id = -1;
  if (frame_host) {
    frame_process_id = frame_host->GetProcess()->GetID();
    frame_routing_id = frame_host->GetRoutingID();
  }
  if (!navigation_handle->IsErrorPage()) {
     FIXME: All the Emit() calls below could potentially result in |this|
     being destroyed (by JS listening for the event and calling
     webContents.destroy()).
    auto url = navigation_handle->GetURL();
    bool is_same_document = navigation_handle->IsSameDocument();
    if (is_same_document) {
      Emit("did-navigate-in-page", url, is_main_frame, frame_process_id,
           frame_routing_id);
    } else {
      const net::HttpResponseHeaders* http_response =
          navigation_handle->GetResponseHeaders();
      std::string http_status_text;
      int http_response_code = -1;
      if (http_response) {
        http_status_text = http_response->GetStatusText();
        http_response_code = http_response->response_code();
      }
      Emit("did-frame-navigate", url, http_response_code, http_status_text,
           is_main_frame, frame_process_id, frame_routing_id);
      if (is_main_frame) {
        Emit("did-navigate", url, http_response_code, http_status_text);
      }