### 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
v8::Local<v8::Value> val,
                     printing::mojom::DuplexMode* out) {
    std::string mode;
    if (ConvertFromV8(isolate, val, &mode)) {
      if (mode == "simplex") {
        *out = printing::mojom::DuplexMode::kSimplex;
        return true;
      }
      if (mode == "longEdge") {
        *out = printing::mojom::DuplexMode::kLongEdge;
        return true;
      }
      if (mode == "shortEdge") {
        *out = printing::mojom::DuplexMode::kShortEdge;
        return true;
      }
    }
    return false;
  }
};
#endif
template <>
struct Converter<WindowOpenDisposition> {
  static v8::Local<v8::Value> ToV8(v8::Isolate* isolate,