### 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.h
#ifndef ELECTRON_SHELL_BROWSER_API_ELECTRON_API_WEB_CONTENTS_H_
#define ELECTRON_SHELL_BROWSER_API_ELECTRON_API_WEB_CONTENTS_H_
#include <map>
#include <memory>
#include <string>
#include <utility>
#include <vector>
#include "base/memory/weak_ptr.h"
#include "base/observer_list.h"
#include "base/observer_list_types.h"
#include "chrome/browser/devtools/devtools_eye_dropper.h"
#include "chrome/browser/devtools/devtools_file_system_indexer.h"
#include "chrome/browser/ui/exclusive_access/exclusive_access_context.h"  
#include "content/common/cursors/webcursor.h"
#include "content/common/frame.mojom.h"
#include "content/public/browser/devtools_agent_host.h"
#include "content/public/browser/keyboard_event_processing_result.h"
#include "content/public/browser/permission_type.h"
#include "content/public/browser/render_widget_host.h"