### 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
frame_subscriber_.reset();
}
void WebContents::StartDrag(const gin_helper::Dictionary& item,
                            gin::Arguments* args) {
  base::FilePath file;
  std::vector<base::FilePath> files;
  if (!item.Get("files", &files) && item.Get("file", &file)) {
    files.push_back(file);
  }
  v8::Local<v8::Value> icon_value;
  if (!item.Get("icon", &icon_value)) {
    gin_helper::ErrorThrower(args->isolate())
        .ThrowError("'icon' parameter is required");
    return;
  }
  NativeImage* icon = nullptr;
  if (!NativeImage::TryConvertNativeImage(args->isolate(), icon_value, &icon) ||
      icon->image().IsEmpty()) {
    return;
  }
   Start dragging.
  if (!files.empty()) {
    base::CurrentThread::ScopedNestableTaskAllower allow;
    DragFileItems(files, icon->image(), ontents()->GetNativeView());
  } else {
    gin_helper::ErrorThrower(args->isolate())
        .ThrowError("Must specify either 'file' or 'files' option");
  }
}
v8::Local<v8::Promise> WebContents::CapturePage(gin::Arguments* args) {