### 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/ui/inspectable_ontents_view.h"
#include "shell/browser/ontents_permission_helper.h"
#include "shell/browser/ontents_preferences.h"
#include "shell/browser/ontents_zoom_controller.h"
#include "shell/browser/web_dialog_helper.h"
#include "shell/browser/web_view_guest_delegate.h"
#include "shell/common/api/electron_api_native_image.h"
#include "shell/common/color_util.h"
#include "shell/common/electron_constants.h"
#include "shell/common/gin_converters/base_converter.h"
#include "shell/common/gin_converters/blink_converter.h"
#include "shell/common/gin_converters/callback_converter.h"
#include "shell/common/gin_converters/content_converter.h"
#include "shell/common/gin_converters/file_path_converter.h"
#include "shell/common/gin_converters/frame_converter.h"
#include "shell/common/gin_converters/gfx_converter.h"
#include "shell/common/gin_converters/gurl_converter.h"
#include "shell/common/gin_converters/image_converter.h"
#include "shell/common/gin_converters/net_converter.h"
#include "shell/common/gin_converters/value_converter.h"
#include "shell/common/gin_helper/dictionary.h"
#include "shell/common/gin_helper/object_template_builder.h"
#include "shell/common/language_util.h"
#include "shell/common/mouse_util.h"
#include "shell/common/node_includes.h"
#include "shell/common/options_switches.h"
#include "shell/common/process_util.h"
#include "shell/common/v8_value_serializer.h"
#include "storage/browser/file_system/isolated_context.h"
#include "third_party/blink/public/common/associated_interfaces/associated_interface_provider.h"