### 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
"Failed to load URL: " + url.possibly_invalid_spec() +
              " with error: " + description,
          "electron");
      Emit("did-fail-load", code, description, url, is_main_frame,
           frame_process_id, frame_routing_id);
    }
  }
  content::NavigationEntry* entry = navigation_handle->GetNavigationEntry();
   This check is needed due to an issue in Chromium
   Check the Chromium issue to keep updated:
   https:bugs.chromium.org/p/chromium/issues/detail?id=1178663
   If a history entry has been made and the forward/back call has been made,
   proceed with setting the new title
  if (entry && (entry->GetTransitionType() & ui::PAGE_TRANSITION_FORWARD_BACK))
    WebContents::TitleWasSet(entry);
}
void WebContents::TitleWasSet(content::NavigationEntry* entry) {
  std::u16string final_title;
  bool explicit_set = true;
  if (entry) {
    auto title = entry->GetTitle();
    auto url = entry->GetURL();
    if (url.SchemeIsFile() && title.empty()) {
      final_title = base::UTF8ToUTF16(url.ExtractFileName());
      explicit_set = false;
    } else {
      final_title = title;
    }
  } else {
    final_title = ontents()->GetTitle();