### 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
}
    if (IsGuest())
      Emit("load-commit", url, is_main_frame);
  } else {
    auto url = navigation_handle->GetURL();
    int code = navigation_handle->GetNetErrorCode();
    auto description = net::ErrorToShortString(code);
    Emit("did-fail-provisional-load", code, description, url, is_main_frame,
         frame_process_id, frame_routing_id);
     Do not emit "did-fail-load" for canceled requests.
    if (code != net::ERR_ABORTED) {
      EmitWarning(
          node::Environment::GetCurrent(JavascriptEnvironment::GetIsolate()),
          "Failed to load URL: " + url.possibly_invalid_spec() +
              " with error: " + description,
          "electron");
      Emit("did-fail-load", code, description, url, is_main_frame,
           frame_process_id, frame_routing_id);
    }
  }
  content::NavigationEntry* entry = navigation_handle->GetNavigationEntry();
   This check is needed due to an issue in Chromium
   Check the Chromium issue to keep updated:
   https:bugs.chromium.org/p/chromium/issues/detail?id=1178663
   If a history entry has been made and the forward/back call has been made,
   proceed with setting the new title
  if (entry && (entry->GetTransitionType() & ui::PAGE_TRANSITION_FORWARD_BACK))
    WebContents::TitleWasSet(entry);
}
void WebContents::TitleWasSet(content::NavigationEntry* entry) {