### 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
.ThrowError("webContents.print(): Invalid print settings specified.");
    return;
  }
  printing::CompletionCallback callback;
  if (args->Length() == 2 && !args->GetNext(&callback)) {
    gin_helper::ErrorThrower(args->isolate())
        .ThrowError("webContents.print(): Invalid optional callback provided.");
    return;
  }
   Set optional silent printing
  bool silent = false;
  options.Get("silent", &silent);
  bool print_background = false;
  options.Get("printBackground", &print_background);
  settings.SetBoolKey(printing::kSettingShouldPrintBackgrounds,
                      print_background);
   Set custom margin settings
  gin_helper::Dictionary margins =
      gin::Dictionary::CreateEmpty(args->isolate());
  if (options.Get("margins", &margins)) {
    printing::mojom::MarginType margin_type =
        printing::mojom::MarginType::kDefaultMargins;
    margins.Get("marginType", &margin_type);
    settings.SetIntKey(printing::kSettingMarginsType,
                       static_cast<int>(margin_type));
    if (margin_type == printing::mojom::MarginType::kCustomMargins) {
      base::Value custom_margins(base::Value::Type::DICTIONARY);
      int top = 0;
      margins.Get("top", &top);
      custom_margins.SetIntKey(printing::kSettingMarginTop, top);