### 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
int bottom = 0;
      margins.Get("bottom", &bottom);
      custom_margins.SetIntKey(printing::kSettingMarginBottom, bottom);
      int left = 0;
      margins.Get("left", &left);
      custom_margins.SetIntKey(printing::kSettingMarginLeft, left);
      int right = 0;
      margins.Get("right", &right);
      custom_margins.SetIntKey(printing::kSettingMarginRight, right);
      settings.SetPath(printing::kSettingMarginsCustom,
                       std::move(custom_margins));
    }
  } else {
    settings.SetIntKey(
        printing::kSettingMarginsType,
        static_cast<int>(printing::mojom::MarginType::kDefaultMargins));
  }
   Set whether to print color or greyscale
  bool print_color = true;
  options.Get("color", &print_color);
  auto const color_model = print_color ? printing::mojom::ColorModel::kColor
                                       : printing::mojom::ColorModel::kGray;
  settings.SetIntKey(printing::kSettingColor, static_cast<int>(color_model));
   Is the orientation landscape or portrait.
  bool landscape = false;
  options.Get("landscape", &landscape);
  settings.SetBoolKey(printing::kSettingLandscape, landscape);
   We set the default to the system's default printer and only update
   if at the Chromium level if the user overrides.
   Printer device name as opened by the OS.