### 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
bool DidAddMessageToConsole(content::WebContents* source,
                              blink::mojom::ConsoleMessageLevel level,
                              const std::u16string& message,
                              int32_t line_no,
                              const std::u16string& source_id) override;
  bool IsWebContentsCreationOverridden(
      content::SiteInstance* source_site_instance,
      content::mojom::WindowContainerType window_container_type,
      const GURL& opener_url,
      const content::mojom::CreateNewWindowParams& params) override;
  content::WebContents* CreateCustomWebContents(
      content::RenderFrameHost* opener,
      content::SiteInstance* source_site_instance,
      bool is_new_browsing_instance,
      const GURL& opener_url,
      const std::string& frame_name,
      const GURL& target_url,
      const content::StoragePartitionId& partition_id,
      content::SessionStorageNamespace* session_storage_namespace) override;
  void WebContentsCreatedWithFullParams(
      content::WebContents* source_contents,
      int opener_render_process_id,
      int opener_render_frame_id,
      const content::mojom::CreateNewWindowParams& params,
      content::WebContents* new_contents) override;
  void AddNewContents(content::WebContents* source,
                      std::unique_ptr<content::WebContents> new_contents,
                      const GURL& target_url,
                      WindowOpenDisposition disposition,