### 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
int request_id,
                            int number_of_matches,
                            const gfx::Rect& selection_rect,
                            int active_match_ordinal,
                            bool final_update) {
  if (!final_update)
    return;
  v8::Isolate* isolate = JavascriptEnvironment::GetIsolate();
  v8::Locker locker(isolate);
  v8::HandleScope handle_scope(isolate);
  gin_helper::Dictionary result = gin::Dictionary::CreateEmpty(isolate);
  result.Set("requestId", request_id);
  result.Set("matches", number_of_matches);
  result.Set("selectionArea", selection_rect);
  result.Set("activeMatchOrdinal", active_match_ordinal);
  result.Set("finalUpdate", final_update);  
  Emit("found-in-page", result.GetHandle());
}
bool WebContents::CheckMediaAccessPermission(
    content::RenderFrameHost* render_frame_host,
    const GURL& security_origin,
    blink::mojom::MediaStreamType type) {
  auto* ontents =
      content::WebContents::FromRenderFrameHost(render_frame_host);
  auto* permission_helper =
      WebContentsPermissionHelper::FromWebContents(ontents);
  return permission_helper->CheckMediaAccessPermission(security_origin, type);
}
void WebContents::RequestMediaAccessPermission(