### 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
#include "shell/browser/extensions/electron_extension_ontents_observer.h"
#endif
#if BUILDFLAG(ENABLE_PRINTING)
#include "chrome/browser/printing/print_view_manager_basic.h"
#include "components/printing/browser/print_manager_utils.h"
#include "printing/backend/print_backend.h"  
#include "printing/mojom/print.mojom.h"
#include "shell/browser/printing/print_preview_message_handler.h"
#if defined(OS_WIN)
#include "printing/backend/win_helper.h"
#endif
#endif
#if BUILDFLAG(ENABLE_COLOR_CHOOSER)
#include "chrome/browser/ui/color_chooser.h"
#endif
#if BUILDFLAG(ENABLE_PICTURE_IN_PICTURE)
#include "chrome/browser/picture_in_picture/picture_in_picture_window_manager.h"
#endif
#if BUILDFLAG(ENABLE_PDF_VIEWER)
#include "components/pdf/browser/pdf_ontents_helper.h"  
#include "shell/browser/electron_pdf_ontents_helper_client.h"
#endif
#ifndef MAS_BUILD
#include "chrome/browser/hang_monitor/hang_crash_dump.h"  
#endif
namespace gin {
#if BUILDFLAG(ENABLE_PRINTING)
template <>
struct Converter<printing::mojom::MarginType> {
  static bool FromV8(v8::Isolate* isolate,