### 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 "third_party/blink/public/common/input/web_input_event.h"
#include "third_party/blink/public/common/messaging/transferable_message_mojom_traits.h"
#include "third_party/blink/public/common/page/page_zoom.h"
#include "third_party/blink/public/mojom/frame/find_in_page.mojom.h"
#include "third_party/blink/public/mojom/frame/fullscreen.mojom.h"
#include "third_party/blink/public/mojom/messaging/transferable_message.mojom.h"
#include "third_party/blink/public/mojom/renderer_preferences.mojom.h"
#include "ui/base/cursor/cursor.h"
#include "ui/base/cursor/mojom/cursor_type.mojom-shared.h"
#include "ui/display/screen.h"
#include "ui/events/base_event_utils.h"
#if BUILDFLAG(ENABLE_OSR)
#include "shell/browser/osr/osr_render_widget_host_view.h"
#include "shell/browser/osr/osr_ontents_view.h"
#endif
#if !defined(OS_MAC)
#include "ui/aura/window.h"
#else
#include "ui/base/cocoa/defaults_utils.h"
#endif
#if defined(OS_LINUX)
#include "ui/views/linux_ui/linux_ui.h"
#endif
#if defined(OS_LINUX) || defined(OS_WIN)
#include "ui/gfx/font_render_params.h"
#endif
#if BUILDFLAG(ENABLE_ELECTRON_EXTENSIONS)
#include "extensions/browser/script_executor.h"
#include "extensions/browser/view_type_utils.h"
#include "extensions/common/mojom/view_type.mojom.h"