### 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
return ontents()->IsCrashed();
}
void WebContents::ForcefullyCrashRenderer() {
  content::RenderWidgetHostView* view =
      ontents()->GetRenderWidgetHostView();
  if (!view)
    return;
  content::RenderWidgetHost* rwh = view->GetRenderWidgetHost();
  if (!rwh)
    return;
  content::RenderProcessHost* rph = rwh->GetProcess();
  if (rph) {
#if defined(OS_LINUX) || defined(OS_CHROMEOS)
     A generic |CrashDumpHungChildProcess()| is not implemented for Linux.
     Instead we send an explicit IPC to crash on the renderer's IO thread.
    rph->ForceCrash();
#else
     Try to generate a crash report for the hung process.
#ifndef MAS_BUILD
    CrashDumpHungChildProcess(rph->GetProcess().Handle());
#endif
    rph->Shutdown(content::RESULT_CODE_HUNG);
#endif
  }
}
void WebContents::SetUserAgent(const std::string& user_agent) {