### 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
promise.RejectWithErrorMessage("takeHeapSnapshot failed");
    return handle;
  }
   This dance with `base::Owned` is to ensure that the interface stays alive
   until the callback is called. Otherwise it would be closed at the end of
   this function.
  auto electron_renderer =
      std::make_unique<mojo::Remote<mojom::ElectronRenderer>>();
  frame_host->GetRemoteInterfaces()->GetInterface(
      electron_renderer->BindNewPipeAndPassReceiver());
  auto* raw_ptr = electron_renderer.get();
  (*raw_ptr)->TakeHeapSnapshot(
      mojo::WrapPlatformFile(base::ScopedPlatformFile(file.TakePlatformFile())),
      base::BindOnce(
          [](mojo::Remote<mojom::ElectronRenderer>* ep,
             gin_helper::Promise<void> promise, bool success) {
            if (success) {
              promise.Resolve();
            } else {
              promise.RejectWithErrorMessage("takeHeapSnapshot failed");
            }
          },
          base::Owned(std::move(electron_renderer)), std::move(promise)));
  return handle;
}
void WebContents::UpdatePreferredSize(content::WebContents* ontents,
                                      const gfx::Size& pref_size) {
  Emit("preferred-size-changed", pref_size);
}
bool WebContents::CanOverscrollContent() {