### 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
#include <set>
#include <string>
#include <unordered_set>
#include <utility>
#include <vector>
#include "base/containers/id_map.h"
#include "base/files/file_util.h"
#include "base/ignore_result.h"
#include "base/json/json_reader.h"
#include "base/no_destructor.h"
#include "base/strings/utf_string_conversions.h"
#include "base/task/current_thread.h"
#include "base/task/post_task.h"
#include "base/task/thread_pool.h"
#include "base/threading/scoped_blocking_call.h"
#include "base/threading/sequenced_task_runner_handle.h"
#include "base/threading/thread_restrictions.h"
#include "base/threading/thread_task_runner_handle.h"
#include "base/values.h"
#include "chrome/browser/browser_process.h"
#include "chrome/browser/ui/exclusive_access/exclusive_access_manager.h"
#include "chrome/browser/ui/views/eye_dropper/eye_dropper.h"
#include "chrome/common/pref_names.h"
#include "components/prefs/pref_service.h"
#include "components/prefs/scoped_user_pref_update.h"
#include "components/security_state/content/content_utils.h"
#include "components/security_state/core/security_state.h"
#include "content/browser/renderer_host/frame_tree_node.h"  
#include "content/browser/renderer_host/render_frame_host_manager.h"  
#include "content/browser/renderer_host/render_widget_host_impl.h"