### 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 (embedder) {
    NativeWindow* owner_window = nullptr;
    auto* relay = NativeWindowRelay::FromWebContents(embedder->ontents());
    if (relay) {
      owner_window = relay->GetNativeWindow();
    }
    if (owner_window)
      SetOwnerWindow(owner_window);
    content::RenderWidgetHostView* rwhv =
        ontents()->GetRenderWidgetHostView();
    if (rwhv) {
      rwhv->Hide();
      rwhv->Show();
    }
  }
}
void WebContents::SetDevToolsWebContents(const WebContents* devtools) {
  if (inspectable_ontents_)
    inspectable_ontents_->SetDevToolsWebContents(devtools->ontents());
}
v8::Local<v8::Value> WebContents::GetNativeView(v8::Isolate* isolate) const {
  gfx::NativeView ptr = ontents()->GetNativeView();
  auto buffer = node::Buffer::Copy(isolate, reinterpret_cast<char*>(&ptr),
                                   sizeof(gfx::NativeView));
  if (buffer.IsEmpty())
    return v8::Null(isolate);
  else
    return buffer.ToLocalChecked();
}
v8::Local<v8::Value> WebContents::DevToolsWebContents(v8::Isolate* isolate) {