### 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 OnExclusiveAccessUserInput() override;
  content::WebContents* GetActiveWebContents() override;
  bool CanUserExitFullscreen() const override;
  bool IsExclusiveAccessBubbleDisplayed() const override;
  bool IsFullscreenForTabOrPending(const content::WebContents* source) override;
  bool TakeFocus(content::WebContents* source, bool reverse) override;
  content::PictureInPictureResult EnterPictureInPicture(
      content::WebContents* web_contents,
      const viz::SurfaceId&,
      const gfx::Size& natural_size) override;
  void ExitPictureInPicture() override;
  
  void DevToolsSaveToFile(const std::string& url,
                          const std::string& content,
                          bool save_as) override;
  void DevToolsAppendToFile(const std::string& url,
                            const std::string& content) override;
  void DevToolsRequestFileSystems() override;
  void DevToolsAddFileSystem(const std::string& type,
                             const base::FilePath& file_system_path) override;
  void DevToolsRemoveFileSystem(
      const base::FilePath& file_system_path) override;
  void DevToolsIndexPath(int request_id,
                         const std::string& file_system_path,
                         const std::string& excluded_folders_message) override;
  void DevToolsStopIndexing(int request_id) override;
  void DevToolsSearchInPath(int request_id,
                            const std::string& file_system_path,
                            const std::string& query) override;
  void DevToolsSetEyeDropperActive(bool active) override;