### 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_WIN)
  
  
  base::ThreadRestrictions::ScopedAllowIO allow_io;
#endif
  scoped_refptr<printing::PrintBackend> print_backend =
      printing::PrintBackend::CreateInstance(
          g_browser_process->GetApplicationLocale());
  std::string printer_name;
  printing::mojom::ResultCode code =
      print_backend->GetDefaultPrinterName(printer_name);
  
  
  if (code != printing::mojom::ResultCode::kSuccess)
    LOG(ERROR) << "Failed to get default printer name";
  
  if (printer_name.empty()) {
    printing::PrinterList printers;
    if (print_backend->EnumeratePrinters(&printers) !=
        printing::mojom::ResultCode::kSuccess)
      return std::make_pair("Failed to enumerate printers", std::u16string());
    if (!printers.empty())
      printer_name = printers.front().printer_name;
  }
  return std::make_pair(std::string(), base::UTF8ToUTF16(printer_name));
}
scoped_refptr<base::TaskRunner> CreatePrinterHandlerTaskRunner() {