### 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
const content::ContextMenuParams& params) {
  Emit("context-menu", std::make_pair(params, &render_frame_host));
  return true;
}
bool WebContents::OnGoToEntryOffset(int offset) {
  GoToOffset(offset);
  return false;
}
void WebContents::FindReply(content::WebContents* ontents,
                            int request_id,
                            int number_of_matches,
                            const gfx::Rect& selection_rect,
                            int active_match_ordinal,
                            bool final_update) {
  if (!final_update)
    return;
  v8::Isolate* isolate = JavascriptEnvironment::GetIsolate();
  v8::Locker locker(isolate);
  v8::HandleScope handle_scope(isolate);
  gin_helper::Dictionary result = gin::Dictionary::CreateEmpty(isolate);
  result.Set("requestId", request_id);
  result.Set("matches", number_of_matches);
  result.Set("selectionArea", selection_rect);
  result.Set("activeMatchOrdinal", active_match_ordinal);
  result.Set("finalUpdate", final_update);  
  Emit("found-in-page", result.GetHandle());
}
void WebContents::RequestKeyboardLock(content::WebContents* ontents,