### 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
if (type_ == Type::kRemote)
    return;
  if (!enable_devtools_)
    return;
  DCHECK(inspectable_ontents_);
  if (!inspectable_ontents_->GetDevToolsWebContents())
    OpenDevTools(nullptr);
  inspectable_ontents_->InspectElement(x, y);
}
void WebContents::InspectSharedWorkerById(const std::string& workerId) {
  if (type_ == Type::kRemote)
    return;
  if (!enable_devtools_)
    return;
  for (const auto& agent_host : content::DevToolsAgentHost::GetOrCreateAll()) {
    if (agent_host->GetType() ==
        content::DevToolsAgentHost::kTypeSharedWorker) {
      if (agent_host->GetId() == workerId) {
        OpenDevTools(nullptr);
        inspectable_ontents_->AttachTo(agent_host);
        break;
      }
    }
  }
}
std::vector<scoped_refptr<content::DevToolsAgentHost>>