### 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 std::string& channel,
    blink::TransferableMessage message,
    content::RenderFrameHost* render_frame_host) {
  v8::Isolate* isolate = JavascriptEnvironment::GetIsolate();
  v8::HandleScope handle_scope(isolate);
  auto wrapped_ports =
      MessagePort::EntanglePorts(isolate, std::move(message.ports));
  v8::Local<v8::Value> message_value =
      electron::DeserializeV8Value(isolate, message);
  EmitWithSender("-ipc-ports", render_frame_host,
                 electron::mojom::ElectronBrowser::InvokeCallback(), false,
                 channel, message_value, std::move(wrapped_ports));
}
void WebContents::MessageSync(
    bool internal,
    const std::string& channel,
    blink::CloneableMessage arguments,
    electron::mojom::ElectronBrowser::MessageSyncCallback callback,
    content::RenderFrameHost* render_frame_host) {
  TRACE_EVENT1("electron", "WebContents::MessageSync", "channel", channel);
   webContents.emit('-ipc-message-sync', new Event(sender, message), internal,
   channel, arguments);
  EmitWithSender("-ipc-message-sync", render_frame_host, std::move(callback),
                 internal, channel, std::move(arguments));
}
void WebContents::MessageTo(int32_t ontents_id,