### 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
const std::string& file_system_path) {
  auto file_system_paths = GetAddedFileSystemPaths(ontents);
  return file_system_paths.find(file_system_path) != file_system_paths.end();
}
void SetBackgroundColor(content::RenderWidgetHostView* rwhv, SkColor color) {
  rwhv->SetBackgroundColor(color);
  static_cast<content::RenderWidgetHostViewBase*>(rwhv)
      ->SetContentBackgroundColor(color);
}
e
#if BUILDFLAG(ENABLE_ELECTRON_EXTENSIONS)
WebContents::Type GetTypeFromViewType(extensions::mojom::ViewType view_type) {
  switch (view_type) {
    case extensions::mojom::ViewType::kExtensionBackgroundPage:
      return WebContents::Type::kBackgroundPage;
    case extensions::mojom::ViewType::kAppWindow:
    case extensions::mojom::ViewType::kComponent:
    case extensions::mojom::ViewType::kExtensionDialog:
    case extensions::mojom::ViewType::kExtensionPopup:
    case extensions::mojom::ViewType::kBackgroundContents:
    case extensions::mojom::ViewType::kExtensionGuest:
    case extensions::mojom::ViewType::kTabContents:
    case extensions::mojom::ViewType::kInvalid:
      return WebContents::Type::kRemote;
  }
}
#endif
WebContents::WebContents(v8::Isolate* isolate,