### 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
Because web preferences don't recognize transparency,
       only set rwhv background color if a color exists
      auto* rwhv = ontents->ontents()->GetRenderWidgetHostView();
      if (rwhv && color.has_value())
        SetBackgroundColor(rwhv, color.value());
    }
  } else {
     Create one if not.
    ontents = WebContents::New(isolate, web_preferences);
  }
  return ontents;
}
 static
WebConbContents::FromID(int32_t id) {
  return GetAllWebContents().Lookup(id);
}
 static
gin::Wo WebContents::kWo = {gin::kEmbedderNativeGin};
e api
e electron
namespace {
using electron::api::GetAllWebContents;
using electron::api::WebContents;
gin::HContents> WebContentsFromID(v8::Isolate* isolate, int32_t id) {
  WebContents* contents = WebContents::FromID(id);
  return contents ? gin::CreateHandle(isolate, contents)
                  : gin::HContents>();
}
gin::HContents> WebContentsFromDevToolsTargetID(
    v8::Isolate* isolate,