### 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 (!inspectable_ontents_)
    return nullptr;
  return inspectable_ontents_->GetDevToolsWebContents();
}
void WebContents::WebContentsDestroyed() {
   Clear the pointer stored in wrapper.
  if (GetAllWebContents().Lookup(id_))
    GetAllWebContents().Remove(id_);
  v8::Isolate* isolate = JavascriptEnvironment::GetIsolate();
  v8::HandleScope scope(isolate);
  v8::Lobject> wrapper;
  if (!GetWrapper(isolate).ToLocal(&wrapper))
    return;
  wrapper->SetAlignedPointerInInternalField(0, nullptr);
  Observe(nullptr);
  Emit("destroyed");
}
void WebContents::NavigationEntryCommitted(
    const content::LoadCommittedDetails& details) {
  Emit("navigation-entry-committed", details.entry->GetURL(),
       details.is_same_document, details.did_replace_entry);
}
bool WebContents::GetBackgroundThrottling() const {
  return background_throttling_;
}
void WebContents::SetBackgroundThrottling(bool allowed) {