### 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
static base::NoDestructor<base::IDMap<WebContents*>> s_all_ontents;
  return *s_all_ontents;
}
void OnCapturePageDone(gin_helper::Promise<gfx::Image> promise,
                       const SkBitmap& bitmap) {
  
  promise.Resolve(gfx::Image::CreateFrom1xBitmap(bitmap));
}
absl::optional<base::TimeDelta> GetCursorBlinkInterval() {
#if defined(OS_MAC)
  base::TimeDelta interval;
  if (ui::TextInsertionCaretBlinkPeriod(&interval))
    return interval;
#elif defined(OS_LINUX)
  if (auto* linux_ui = views::LinuxUI::instance())
    return linux_ui->GetCursorBlinkInterval();
#elif defined(OS_WIN)
  const auto system_msec = ::GetCaretBlinkTime();
  if (system_msec != 0) {
    return (system_msec == INFINITE) ? base::TimeDelta()
                                     : base::Milliseconds(system_msec);
  }
#endif
  return absl::nullopt;
}
#if BUILDFLAG(ENABLE_PRINTING)
bool IsDeviceNameValid(const std::u16string& device_name) {