### 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:** 5.0.7
* **Operating System:** Windows 10

### Expected Behavior
When I navigate a `BrowserView` to a page that has `beforeunload` and `unload` handlers, I expect that closing the window will cause them to fire.

### Actual Behavior
They do not fire when closing the window.

Notably, if I don't use `BrowserView` and just navigate the `BrowserWindow` to the page, those handlers will execute.

### To Reproduce

`npm start` the code below (in main.js). In the devtools that opens up, run `window.onbeforeunload = () => true;`, then close the window. In the working case, the window will not close. In the broken case that uses `BrowserView`, it will.

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

function brokenCase() {
	const mainWindow = new BrowserWindow();
	const view = new BrowserView();
	mainWindow.setBrowserView(view);
	view.webContents.loadURL("https://google.com");
	view.webContents.openDevTools();
}

function workingCase() {
	const mainWindow = new BrowserWindow();
	mainWindow.webContents.loadURL("https://google.com");
	mainWindow.webContents.openDevTools();
}

app.on('ready', brokenCase)
```

Maybe related to [this comment](https://github.com/electron/electron/issues/19194#issuecomment-544407723)
shell/browser/api/electron_api_web_contents.cc
const std::string& channel,
    blink::TransferableMessage message,
    content::RenderFrameHost* render_frame_host) {
  v8::Isolate* isolate = JavascriptEnvironment::GetIsolate();
  v8::HandleScope handle_scope(isolate);
  auto wrapped_ports =
      MessagePort::EntanglePorts(isolate, std::move(message.ports));
  v8::Local<v8::Value> message_value =
      electron::DeserializeV8Value(isolate, message);
  EmitWithSender("-ipc-ports", render_frame_host,
                 electron::mojom::ElectronBrowser::InvokeCallback(), false,
                 channel, message_value, std::move(wrapped_ports));
}
void WebContents::MessageSync(
    bool internal,
    const std::string& channel,
    blink::CloneableMessage arguments,
    electron::mojom::ElectronBrowser::MessageSyncCallback callback,
    content::RenderFrameHost* render_frame_host) {
  TRACE_EVENT1("electron", "WebContents::MessageSync", "channel", channel);
   webContents.emit('-ipc-message-sync', new Event(sender, message), internal,
   channel, arguments);
  EmitWithSender("-ipc-message-sync", render_frame_host, std::move(callback),
                 internal, channel, std::move(arguments));
}
void WebContents::MessageTo(bool internal,