### 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
#if defined(OS_MAC)
  base::ScopedCFTypeRef<CFStringRef> new_printer_id(
      base::SysUTF16ToCFStringRef(device_name));
  PMPrinter new_printer = PMPrinterCreateFromPrinterID(new_printer_id.get());
  bool printer_exists = new_printer != nullptr;
  PMRelease(new_printer);
  return printer_exists;
#elif defined(OS_WIN)
  printing::ScopedPrinterHandle printer;
  return printer.OpenPrinterWithName(base::as_wcstr(device_name));
#else
  return true;
#endif
}
std::pair<std::string, std::u16string> GetDefaultPrinterAsync() {