### 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
auto handle = FromOrCreate(
      isolate, inspectable_ontents_->GetDevToolsWebContents());
  devtools_ontents_.Reset(isolate, handle.ToV8());
   Set inspected tabID.
  base::Value tab_id(ID());
  inspectable_ontents_->CallClientFunction("DevToolsAPI.setInspectedTabId",
                                                &tab_id, nullptr, nullptr);
   Inherit owner window in devtools when it doesn't have one.
  auto* devtools = inspectable_ontents_->GetDevToolsWebContents();
  bool has_window = devtools->GetUserData(NativeWindowRelay::UserDataKey());
  if (owner_window() && !has_window)
    handle->SetOwnerWindow(devtools, owner_window());
  Emit("devtools-opened");
}
void WebContents::DevToolsClosed() {
  v8::Isolate* isolate = JavascriptEnvironment::GetIsolate();
  v8::Locker locker(isolate);
  v8::HandleScope handle_scope(isolate);
  devtools_ontents_.Reset();
  Emit("devtools-closed");
}
void WebContents::DevToolsResized() {
  for (ExtendedWebContentsObserver& observer : observers_)
    observer.OnDevToolsResized();
}
void WebContents::SetOwnerWindow(NativeWindow* owner_window) {
  SetOwnerWindow(GetWebContents(), owner_window);
}
void WebContents::SetOwnerWindow(content::WebContents* ontents,
                                 NativeWindow* owner_window) {