### 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
base::ProcessId plugin_pid) {
#if BUILDFLAG(ENABLE_PLUGINS)
  content::WebPluginInfo info;
  auto* plugin_service = content::PluginService::GetInstance();
  plugin_service->GetPluginInfoByPath(plugin_path, &info);
  Emit("plugin-crashed", info.name, info.version);
#endif  
}
void WebContents::MediaStartedPlaying(const MediaPlayerInfo& video_type,
                                      const content::MediaPlayerId& id) {
  Emit("media-started-playing");
}
void WebContents::MediaStoppedPlaying(
    const MediaPlayerInfo& video_type,
    const content::MediaPlayerId& id,
    content::WebContentsObserver::MediaStoppedReason reason) {
  Emit("media-paused");
}
void WebContents::DidChangeThemeColor() {
  auto theme_color = ontents()->GetThemeColor();
  if (theme_color) {
    Emit("did-change-theme-color", electron::ToRGBHex(theme_color.value()));
  } else {
    Emit("did-change-theme-color", nullptr);
  }
}
void WebContents::DidAcquireFullscreen(content::RenderFrameHost* rfh) {