### 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
double factor) {
  if (factor < std::numeric_limits<double>::epsilon()) {
    thrower.ThrowError("'zoomFactor' must be a double greater than 0.0");
    return;
  }
  auto level = blink::PageZoomFactorToZoomLevel(factor);
  SetZoomLevel(level);
}
double WebContents::GetZoomFactor() const {
  auto level = GetZoomLevel();
  return blink::PageZoomLevelToZoomFactor(level);
}
void WebContents::SetTemporaryZoomLevel(double level) {
  zoom_controller_->SetTemporaryZoomLevel(level);
}
void WebContents::DoGetZoomLevel(
    electron::mojom::ElectronBrowser::DoGetZoomLevelCallback callback) {
  std::move(callback).Run(GetZoomLevel());
}
std::vector<base::FilePath> WebContents::GetPreloadPaths() const {
  auto result = SessionPreferences::GetValidPreloads(GetBrowserContext());
  if (auto* web_preferences = WebContentsPreferences::From(ontents())) {
    base::FilePath preload;
    if (web_preferences->GetPreloadPath(&preload)) {
      result.emplace_back(preload);
    }
  }
  return result;
}
v8::Local<v8::Value> WebContents::GetLastWebPreferences(