### 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 "shell/browser/extensions/electron_extension_ontents_observer.h"
#endif
#if BUILDFLAG(ENABLE_PRINTING)
#include "components/printing/browser/print_manager_utils.h"
#include "printing/backend/print_backend.h"  
#include "printing/mojom/print.mojom.h"
#include "shell/browser/printing/print_preview_message_handler.h"
#include "shell/browser/printing/print_view_manager_electron.h"
#if defined(OS_WIN)
#include "printing/backend/win_helper.h"
#endif
#endif
#if BUILDFLAG(ENABLE_PICTURE_IN_PICTURE)
#include "chrome/browser/picture_in_picture/picture_in_picture_window_manager.h"
#endif
#if BUILDFLAG(ENABLE_PDF_VIEWER)
#include "components/pdf/browser/pdf_ontents_helper.h"  
#include "shell/browser/electron_pdf_ontents_helper_client.h"
#endif
#if BUILDFLAG(ENABLE_PLUGINS)
#include "content/public/browser/plugin_service.h"
#endif
#ifndef MAS_BUILD
#include "chrome/browser/hang_monitor/hang_crash_dump.h"  
#endif
namespace gin {
#if BUILDFLAG(ENABLE_PRINTING)
template <>
struct Converter<printing::mojom::MarginType> {
  static bool FromV8(v8::Isolate* isolate,