### 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
.SetMethod("getType", &WebContents::GetType)
      .SetMethod("_getPreloadPaths", &WebContents::GetPreloadPaths)
      .SetMethod("getWebPreferences", &WebContents::GetWebPreferences)
      .SetMethod("getLastWebPreferences", &WebContents::GetLastWebPreferences)
      .SetMethod("getOwnerBrowserWindow", &WebContents::GetOwnerBrowserWindow)
      .SetMethod("inspectServiceWorker", &WebContents::InspectServiceWorker)
      .SetMethod("inspectSharedWorker", &WebContents::InspectSharedWorker)
      .SetMethod("inspectSharedWorkerById",
                 &WebContents::InspectSharedWorkerById)
      .SetMethod("getAllSharedWorkers", &WebContents::GetAllSharedWorkers)
#if BUILDFLAG(ENABLE_PRINTING)
      .SetMethod("_print", &WebContents::Print)
      .SetMethod("_printToPDF", &WebContents::PrintToPDF)
#endif
      .SetMethod("_setNextChildWebPreferences",
                 &WebContents::SetNextChildWebPreferences)
      .SetMethod("addWorkSpace", &WebContents::AddWorkSpace)
      .SetMethod("removeWorkSpace", &WebContents::RemoveWorkSpace)
      .SetMethod("showDefinitionForSelection",
                 &WebContents::ShowDefinitionForSelection)
      .SetMethod("copyImageAt", &WebContents::CopyImageAt)
      .SetMethod("capturePage", &WebContents::CapturePage)
      .SetMethod("setEmbedder", &WebContents::SetEmbedder)
      .SetMethod("setDevToolsWebContents", &WebContents::SetDevToolsWebContents)
      .SetMethod("getNativeView", &WebContents::GetNativeView)
      .SetMethod("incrementCapturerCount", &WebContents::IncrementCapturerCount)
      .SetMethod("decrementCapturerCount", &WebContents::DecrementCapturerCount)
      .SetMethod("isBeingCaptured", &WebContents::IsBeingCaptured)
      .SetMethod("setWebRTCIPHandlingPolicy",
                 &WebContents::SetWebRTCIPHandlingPolicy)