### 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
weak_factory_.GetWeakPtr(), request_id, file_system_path),
              base::BindRepeating(&WebContents::OnDevToolsIndexingWorked,
                                  weak_factory_.GetWeakPtr(), request_id,
                                  file_system_path),
              base::BindRepeating(&WebContents::OnDevToolsIndexingDone,
                                  weak_factory_.GetWeakPtr(), request_id,
                                  file_system_path)));
}
void WebContents::DevToolsStopIndexing(int request_id) {
  auto it = devtools_indexing_jobs_.find(request_id);
  if (it == devtools_indexing_jobs_.end())
    return;
  it->second->Stop();
  devtools_indexing_jobs_.erase(it);
}
void WebContents::DevToolsSearchInPath(int request_id,
                                       const std::string& file_system_path,
                                       const std::string& query) {
  if (!IsDevToolsFileSystemAdded(GetDevToolsWebContents(), file_system_path)) {
    OnDevToolsSearchCompleted(request_id, file_system_path,
                              std::vector<std::string>());
    return;
  }
  devtools_file_system_indexer_->SearchInPath(
      file_system_path, query,
      base::BindRepeating(&WebContents::OnDevToolsSearchCompleted,
                          weak_factory_.GetWeakPtr(), request_id,
                          file_system_path));
}
void WebContents::DevToolsSetEyeDropperActive(bool active) {