### 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
base::ScopedBlockingCall scoped_blocking_call(FROM_HERE,
                                                base::BlockingType::WILL_BLOCK);
  DCHECK(!path.empty());
  base::AppendToFile(path, content);
}
PrefService* GetPrefService(content::WebContents* ontents) {
  auto* context = ontents->GetBrowserContext();
  return static_cast<electron::ElectronBrowserContext*>(context)->prefs();
}
std::map<std::string, std::string> GetAddedFileSystemPaths(
    content::WebContents* ontents) {
  auto* pref_service = GetPrefService(ontents);
  const base::DictionaryValue* file_system_paths_value =
      pref_service->GetDictionary(prefs::kDevToolsFileSystemPaths);
  std::map<std::string, std::string> result;
  if (file_system_paths_value) {
    base::DictionaryValue::Iterator it(*file_system_paths_value);
    for (; !it.IsAtEnd(); it.Advance()) {
      std::string type =
          it.value().is_string() ? it.value().GetString() : std::string();
      result[it.key()] = type;
    }
  }
  return result;
}
bool IsDevToolsFileSystemAdded(content::WebContents* ontents,