### 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& content) {
  auto it = saved_files_.find(url);
  if (it == saved_files_.end())
    return;
   Notify DevTools.
  base::Value url_value(url);
  inspectable_ontents_->CallClientFunction("DevToolsAPI.appendedToURL",
                                                &url_value, nullptr, nullptr);
  file_task_runner_->PostTask(
      FROM_HERE, base::BindOnce(&AppendToFile, it->second, content));
}
void WebContents::DevToolsRequestFileSystems() {