### 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
std::string target_id) {
  auto agent_host = content::DevToolsAgentHost::GetForId(target_id);
  WebContents* contents =
      agent_host ? WebContents::From(agent_host->GetWebContents()) : nullptr;
  return contents ? gin::CreateHandle(isolate, contents)
                  : gin::HContents>();
}
std::vector<gin::HContents>> GetAllWebContentsAsV8(
    v8::Isolate* isolate) {
  std::vector<gin::HContents>> list;
  for (auto iter = base::IDMap<WebContents*>::iterator(&GetAllWebContents());
       !iter.IsAtEnd(); iter.Advance()) {
    list.push_back(gin::CreateHandle(isolate, iter.GetCurrentValue()));
  }
  return list;
}
void Initialize(v8::Lobject> exports,
                v8::Local<v8::Value> unused,
                v8::Local<v8::Context> context,
                void* priv) {
  v8::Isolate* isolate = context->GetIsolate();
  gin_helper::Dictionary dict(isolate, exports);
  dict.Set("WebContents", WebContents::GetConstructor(context));
  dict.SetMethod("fromId", &WebContentsFromID);
  dict.SetMethod("fromDevToolsTargetId", &WebContentsFromDevToolsTargetID);
  dict.SetMethod("getAllWebContents", &GetAllWebContentsAsV8);
}
e
NODULE_CONTEXT_AWARE(electron_browser_ontents, Initialize)