### 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
#include "content/public/browser/web_contents.h"
#include "content/public/browser/web_contents_delegate.h"
#include "content/public/browser/web_contents_observer.h"
#include "electron/buildflags/buildflags.h"
#include "electron/shell/common/api/api.mojom.h"
#include "gin/handle.h"
#include "gin/wrappable.h"
#include "mojo/public/cpp/bindings/receiver_set.h"
#include "printing/buildflags/buildflags.h"
#include "shell/browser/api/frame_subscriber.h"
#include "shell/browser/api/save_page_handler.h"
#include "shell/browser/event_emitter_mixin.h"
#include "shell/browser/extended_web_contents_observer.h"
#include "shell/browser/ui/inspectable_web_contents.h"
#include "shell/browser/ui/inspectable_web_contents_delegate.h"
#include "shell/browser/ui/inspectable_web_contents_view_delegate.h"
#include "shell/common/gin_helper/cleaned_up_at_exit.h"
#include "shell/common/gin_helper/constructible.h"
#include "shell/common/gin_helper/error_thrower.h"
#include "shell/common/gin_helper/pinnable.h"
#include "ui/base/models/image_model.h"
#include "ui/gfx/image/image.h"
#if BUILDFLAG(ENABLE_PRINTING)
#include "shell/browser/printing/print_preview_message_handler.h"
#include "shell/browser/printing/print_view_manager_electron.h"
#endif
#if BUILDFLAG(ENABLE_ELECTRON_EXTENSIONS)
#include "extensions/common/mojom/view_type.mojom.h"
namespace extensions {
class ScriptExecutor;