### 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
content::ChildProcessSecurityPolicy::GetInstance()->GrantCommitOrigin(
      ontents()->GetMainFrame()->GetProcess()->GetID(),
      url::Origin::Create(url));
}
void WebContents::NotifyUserActivation() {
  content::RenderFrameHost* frame = ontents()->GetMainFrame();
  if (frame)
    frame->NotifyUserActivation(
        blink::mojom::UserActivationNotificationType::kInteraction);
}
v8::Local<v8::Promise> WebContents::TakeHeapSnapshot(
    v8::Isolate* isolate,
    const base::FilePath& file_path) {
  gin_helper::Promise<void> promise(isolate);
  v8::Local<v8::Promise> handle = promise.GetHandle();
  base::ThreadRestrictions::ScopedAllowIO allow_io;
  base::File file(file_path,
                  base::File::FLAG_CREATE_ALWAYS | base::File::FLAG_WRITE);
  if (!file.IsValid()) {
    promise.RejectWithErrorMessage("takeHeapSnapshot failed");
    return handle;
  }
  auto* frame_host = ontents()->GetMainFrame();
  if (!frame_host) {
    promise.RejectWithErrorMessage("takeHeapSnapshot failed");
    return handle;
  }
  if (!frame_host->IsRenderFrameCreated()) {