### 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
: ontents(contents) {}
  base::WeakPtr<WebContents> ontents;
};
const void* kElectronApiWebContentsKey = &kElectronApiWebContentsKey;
const char kRootName[] = "<root>";
struct FileSystem {
  FileSystem() = default;
  FileSystem(const std::string& type,
             const std::string& file_system_name,
             const std::string& root_url,
             const std::string& file_system_path)
      : type(type),
        file_system_name(file_system_name),
        root_url(root_url),
        file_system_path(file_system_path) {}
  std::string type;
  std::string file_system_name;
  std::string root_url;
  std::string file_system_path;
};
std::string RegisterFileSystem(content::WebContents* ontents,