### 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 "gin/arguments.h"
#include "gin/data_object_builder.h"
#include "gin/handle.h"
#include "gin/object_template_builder.h"
#include "gin/wrappable.h"
#include "mojo/public/cpp/bindings/associated_remote.h"
#include "mojo/public/cpp/bindings/pending_receiver.h"
#include "mojo/public/cpp/bindings/remote.h"
#include "mojo/public/cpp/system/platform_handle.h"
#include "ppapi/buildflags/buildflags.h"
#include "printing/buildflags/buildflags.h"
#include "services/service_manager/public/cpp/interface_provider.h"
#include "shell/browser/api/electron_api_browser_window.h"
#include "shell/browser/api/electron_api_debugger.h"
#include "shell/browser/api/electron_api_session.h"
#include "shell/browser/api/electron_api_web_frame_main.h"
#include "shell/browser/api/message_port.h"
#include "shell/browser/browser.h"
#include "shell/browser/child_ontents_tracker.h"
#include "shell/browser/electron_autofill_driver_factory.h"
#include "shell/browser/electron_browser_client.h"
#include "shell/browser/electron_browser_context.h"
#include "shell/browser/electron_browser_main_parts.h"
#include "shell/browser/electron_javascript_dialog_manager.h"
#include "shell/browser/electron_navigation_throttle.h"
#include "shell/browser/native_window.h"
#include "shell/browser/session_preferences.h"
#include "shell/browser/ui/drag_util.h"
#include "shell/browser/ui/file_dialog.h"
#include "shell/browser/ui/inspectable_ontents.h"