### 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
if (is_same_document) {
      Emit("did-navigate-in-page", url, is_main_frame, frame_process_id,
           frame_routing_id);
    } else {
      const net::HttpResponseHeaders* http_response =
          navigation_handle->GetResponseHeaders();
      std::string http_status_text;
      int http_response_code = -1;
      if (http_response) {
        http_status_text = http_response->GetStatusText();
        http_response_code = http_response->response_code();
      }
      Emit("did-frame-navigate", url, http_response_code, http_status_text,
           is_main_frame, frame_process_id, frame_routing_id);
      if (is_main_frame) {
        Emit("did-navigate", url, http_response_code, http_status_text);
      }
    }
    if (IsGuest())
      Emit("load-commit", url, is_main_frame);
  } else {
    auto url = navigation_handle->GetURL();
    int code = navigation_handle->GetNetErrorCode();
    auto description = net::ErrorToShortString(code);
    Emit("did-fail-provisional-load", code, description, url, is_main_frame,
         frame_process_id, frame_routing_id);
     Do not emit "did-fail-load" for canceled requests.
    if (code != net::ERR_ABORTED) {
      EmitWarning(
          node::Environment::GetCurrent(JavascriptEnvironment::GetIsolate()),