### 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
auto* const host = ontents()->GetMainFrame();
  if (host)
    host->CopyImageAt(x, y);
}
void WebContents::Focus() {
   Focusing on WebContents does not automatically focus the window on macOS
   and Linux, do it manually to match the behavior on Windows.
#if defined(OS_MAC) || defined(OS_LINUX)
  if (owner_window())
    owner_window()->Focus(true);
#endif
  ontents()->Focus();
}
#if !defined(OS_MAC)
bool WebContents::IsFocused() const {
  auto* view = ontents()->GetRenderWidgetHostView();
  if (!view)
    return false;
  if (GetType() != Type::kBackgroundPage) {
    auto* window = ontents()->GetNativeView()->GetToplevelWindow();
    if (window && !window->IsVisible())
      return false;
  }
  return view->HasFocus();
}
#endif
void WebContents::SendInputEvent(v8::Isolate* isolate,