### 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 a feature request that matches the one I want to file, without success.

### Problem Description
When working with BrowserViews, there's no way to listen to focus changes on either BrowserView itself or its WebContents.

### Proposed Solution
On BrowserWindow, there are `focus` and `blur` events. The same events need to be emitted on BrowserView/WebContents.
i.e. : 
```
browserView.on('focus', () => {
    // view is focused, do stuff
});
```
or  
```
browserView.webContents.on('focus', () => {
    // contents are focused, do stuff
});
```

### Alternatives Considered
I can't think of a good way of working around this lack of functionality
shell/browser/api/electron_api_web_contents.cc
}
   Calling LoadURLWithParams() can trigger JS which destroys |this|.
  auto weak_this = GetWeakPtr();
  params.transition_type = ui::PageTransitionFromInt(
      ui::PAGE_TRANSITION_TYPED | ui::PAGE_TRANSITION_FROM_ADDRESS_BAR);
  params.override_user_agent = content::NavigationController::UA_OVERRIDE_TRUE;
   Discord non-committed entries to ensure that we don't re-use a pending
   entry
  ontents()->GetController().DiscardNonCommittedEntries();
  ontents()->GetController().LoadURLWithParams(params);
   ⚠️WARNING!⚠️
   LoadURLWithParams() triggers JS events which can call destroy() on |this|.
   It's not safe to assume that |this| points to valid memory at this point.
  if (!weak_this || !ontents())
    return;
   Required to make beforeunload handler work.
  NotifyUserActivation();
}
 TODO(MarshallOfSound): Figure out what we need to do with post data here, I
 believe the default behavior when we pass "true" is to phone out to the
 delegate and then the controller expects this method to be called again with
 "false" if the user approves the reload.  For now this would result in
 ".reload()" calls on POST data domains failing silently.  Passing false would
 result in them succeeding, but reposting which although more correct could be
 considering a breaking change.
void WebContents::Reload() {
  ontents()->GetController().Reload(content::ReloadType::NORMAL,
                                         /* check_for_repost */ true);
}
void WebContents::ReloadIgnoringCache() {