### 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 "content/browser/renderer_host/render_widget_host_view_base.h"  
#include "content/public/browser/child_process_security_policy.h"
#include "content/public/browser/context_menu_params.h"
#include "content/public/browser/desktop_media_id.h"
#include "content/public/browser/desktop_streams_registry.h"
#include "content/public/browser/download_request_utils.h"
#include "content/public/browser/favicon_status.h"
#include "content/public/browser/file_select_listener.h"
#include "content/public/browser/native_web_keyboard_event.h"
#include "content/public/browser/navigation_details.h"
#include "content/public/browser/navigation_entry.h"
#include "content/public/browser/navigation_handle.h"
#include "content/public/browser/render_frame_host.h"
#include "content/public/browser/render_process_host.h"
#include "content/public/browser/render_view_host.h"
#include "content/public/browser/render_widget_host.h"
#include "content/public/browser/render_widget_host_view.h"
#include "content/public/browser/service_worker_context.h"
#include "content/public/browser/site_instance.h"
#include "content/public/browser/storage_partition.h"
#include "content/public/browser/ontents.h"
#include "content/public/common/referrer_type_converters.h"
#include "content/public/common/webplugininfo.h"
#include "electron/buildflags/buildflags.h"
#include "electron/shell/common/api/api.mojom.h"
#include "gin/arguments.h"
#include "gin/data_object_builder.h"
#include "gin/handle.h"
#include "gin/object_template_builder.h"
#include "gin/wrappable.h"