### 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::CloneableMessage arguments,
                          content::RenderFrameHost* render_frame_host) {
  TRACE_EVENT1("electron", "WebContents::Message", "channel", channel);
   webContents.emit('-ipc-message', new Event(), internal, channel,
   arguments);
  EmitWithSender("-ipc-message", render_frame_host,
                 electron::mojom::ElectronBrowser::InvokeCallback(), internal,
                 channel, std::move(arguments));
}
void WebContents::Invoke(
    bool internal,
    const std::string& channel,
    blink::CloneableMessage arguments,
    electron::mojom::ElectronBrowser::InvokeCallback callback,
    content::RenderFrameHost* render_frame_host) {
  TRACE_EVENT1("electron", "WebContents::Invoke", "channel", channel);
   webContents.emit('-ipc-invoke', new Event(), internal, channel, arguments);
  EmitWithSender("-ipc-invoke", render_frame_host, std::move(callback),
                 internal, channel, std::move(arguments));
}
void WebContents::OnFirstNonEmptyLayout(
    content::RenderFrameHost* render_frame_host) {
  if (render_frame_host == ontents()->GetMainFrame()) {
    Emit("ready-to-show");
  }
}
void WebContents::ReceivePostMessage(