### 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
auto* ontents = GetWebContents();
  if (!ontents)
    return;
  if (active) {
    eye_dropper_ = std::make_unique<DevToolsEyeDropper>(
        ontents, base::BindRepeating(&WebContents::ColorPickedInEyeDropper,
                                          base::Unretained(this)));
  } else {
    eye_dropper_.reset();
  }
}
void WebContents::ColorPickedInEyeDropper(int r, int g, int b, int a) {
  base::DictionaryValue color;
  color.SetInteger("r", r);
  color.SetInteger("g", g);
  color.SetInteger("b", b);
  color.SetInteger("a", a);
  inspectable_ontents_->CallClientFunction(
      "DevToolsAPI.eyeDropperPickedColor", &color, nullptr, nullptr);
}
#if defined(TOOLKIT_VIEWS) && !defined(OS_MAC)
ui::ImageModel WebContents::GetDevToolsWindowIcon() {
  return owner_window() ? owner_window()->GetWindowAppIcon() : ui::ImageModel{};
}
#endif
#if defined(OS_LINUX)
void WebContents::GetDevToolsWindowWMClass(std::string* name,