### 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
const std::string& webrtc_ip_handling_policy) {
  if (GetWebRTCIPHandlingPolicy() == webrtc_ip_handling_policy)
    return;
  ontents()->GetMutableRendererPrefs()->webrtc_ip_handling_policy =
      webrtc_ip_handling_policy;
  ontents()->SyncRendererPrefs();
}
std::string WebContents::GetMediaSourceID(
    content::WebContents* request_ontents) {
  auto* frame_host = ontents()->GetMainFrame();
  if (!frame_host)
    return std::string();
  content::DesktopMediaID media_id(
      content::DesktopMediaID::TYPE_WEB_CONTENTS,
      content::DesktopMediaID::kNullId,
      content::WebContentsMediaCaptureId(frame_host->GetProcess()->GetID(),
                                         frame_host->GetRoutingID()));
  auto* request_frame_host = request_ontents->GetMainFrame();
  if (!request_frame_host)
    return std::string();
  std::string id =
      content::DesktopStreamsRegistry::GetInstance()->RegisterStream(
          request_frame_host->GetProcess()->GetID(),
          request_frame_host->GetRoutingID(),
          url::Origin::Create(request_frame_host->GetLastCommittedURL()
                                  .DeprecatedGetOriginAsURL()),
          media_id, "", content::kRegistryStreamTypeTab);
  return id;
}
bool WebContents::IsCrashed() const {