### 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
v8::Lobject> wrapper;
  if (!GetWrapper(isolate).ToLocal(&wrapper))
    return;
  wrapper->SetAlignedPointerInInternalField(0, nullptr);
   Tell WebViewGuestDelegate that the WebContents has been destroyed.
  if (guest_delegate_)
    guest_delegate_->WillDestroy();
  Observe(nullptr);
  Emit("destroyed");
}
void WebContents::NavigationEntryCommitted(
    const content::LoadCommittedDetails& details) {
  Emit("navigation-entry-committed", details.entry->GetURL(),
       details.is_same_document, details.did_replace_entry);
}
bool WebContents::GetBackgroundThrottling() const {
  return background_throttling_;
}
void WebContents::SetBackgroundThrottling(bool allowed) {
  background_throttling_ = allowed;
  auto* rfh = ontents()->GetMainFrame();
  if (!rfh)
    return;
  auto* rwhv = rfh->GetView();
  if (!rwhv)
    return;
  auto* rwh_impl =
      static_cast<content::RenderWidgetHostImpl*>(rwhv->GetRenderWidgetHost());
  if (!rwh_impl)
    return;