### 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.h
namespace electron {
class ElectronBrowserContext;
class ElectronJavaScriptDialogManager;
class InspectableWebContents;
class WebContentsZoomController;
class WebViewGuestDelegate;
class FrameSubscriber;
class WebDialogHelper;
class NativeWindow;
#if BUILDFLAG(ENABLE_OSR)
class OffScreenRenderWidgetHostView;
class OffScreenWebContentsView;
#endif
namespace api {
using DevicePermissionMap = std::map<
    int,
    std::map<content::PermissionType,
             std::map<url::Origin, std::vector<std::unique_ptr<base::Value>>>>>;
class WebContents : public ExclusiveAccessContext,
                    public gin::Wrappable<WebContents>,
                    public gin_helper::EventEmitterMixin<WebContents>,
                    public gin_helper::Constructible<WebContents>,
                    public gin_helper::Pinnable<WebContents>,
                    public gin_helper::CleanedUpAtExit,
                    public content::WebContentsObserver,
                    public content::WebContentsDelegate,
                    public InspectableWebContentsDelegate,
                    public InspectableWebContentsViewDelegate {
 public:
  enum class Type {