### 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/ontents_preferences.h"
#include "shell/browser/ontents_zoom_controller.h"
#include "shell/browser/web_view_guest_delegate.h"
#include "shell/browser/web_view_manager.h"
#include "shell/common/api/electron_api_native_image.h"
#include "shell/common/api/electron_bindings.h"
#include "shell/common/color_util.h"
#include "shell/common/electron_constants.h"
#include "shell/common/gin_converters/base_converter.h"
#include "shell/common/gin_converters/blink_converter.h"
#include "shell/common/gin_converters/callback_converter.h"
#include "shell/common/gin_converters/content_converter.h"
#include "shell/common/gin_converters/file_path_converter.h"
#include "shell/common/gin_converters/frame_converter.h"
#include "shell/common/gin_converters/gfx_converter.h"
#include "shell/common/gin_converters/gurl_converter.h"
#include "shell/common/gin_converters/image_converter.h"
#include "shell/common/gin_converters/net_converter.h"
#include "shell/common/gin_converters/value_converter.h"
#include "shell/common/gin_helper/dictionary.h"
#include "shell/common/gin_helper/object_template_builder.h"
#include "shell/common/language_util.h"
#include "shell/common/mouse_util.h"
#include "shell/common/node_includes.h"
#include "shell/common/options_switches.h"
#include "shell/common/process_util.h"
#include "shell/common/v8_value_serializer.h"
#include "storage/browser/file_system/isolated_context.h"
#include "third_party/abseil-cpp/absl/types/optional.h"
#include "third_party/blink/public/common/associated_interfaces/associated_interface_provider.h"