### 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
void WebContentsDestroyed() override;
  void NavigationEntryCommitted(
      const content::LoadCommittedDetails& load_details) override;
  void TitleWasSet(content::NavigationEntry* entry) override;
  void DidUpdateFaviconURL(
      content::RenderFrameHost* render_frame_host,
      const std::vector<blink::mojom::FaviconURLPtr>& urls) override;
  void PluginCrashed(const base::FilePath& plugin_path,
                     base::ProcessId plugin_pid) override;
  void MediaStartedPlaying(const MediaPlayerInfo& video_type,
                           const content::MediaPlayerId& id) override;
  void MediaStoppedPlaying(
      const MediaPlayerInfo& video_type,
      const content::MediaPlayerId& id,
      content::WebContentsObserver::MediaStoppedReason reason) override;
  void DidChangeThemeColor() override;
  void OnCursorChanged(const content::WebCursor& cursor) override;
  void DidAcquireFullscreen(content::RenderFrameHost* rfh) override;
  
  void DevToolsReloadPage() override;
  
  void DevToolsFocused() override;
  void DevToolsOpened() override;
  void DevToolsClosed() override;
  void DevToolsResized() override;
  ElectronBrowserContext* GetBrowserContext() const;
  void OnElectronBrowserConnectionError();
#if BUILDFLAG(ENABLE_OSR)
  OffScreenWebContentsView* GetOffScreenWebContentsView() const;
  OffScreenRenderWidgetHostView* GetOffScreenRenderWidgetHostView() const;