From the official Chromium blog:
Chrome 10: Out-of-date plug-in warnings
As we previously mentioned, we believe that some of the most significant opportunities to increase user security revolve around plugins. We’ve made a number of improvements in this area, including actively encouraging users to update their plug-ins to the most secure version. Chrome now detects when a plug-in is out of date and blocks it with a simple infobar. This infobar helps guide the user towards updating their plug-in with the latest security fixes.
I'm glad to have contributed to the implementation of this feature -- a number of core Chrome engineers helped make it a reality. As for the secbrowsing extension, you don't need to, but you can keep it installed. It will continue to let you know when one of your plugins is out of date, even if you are not using it (In Chrome, the warning only appears when a website you visit requires one of your plugins that is out of date). This might be helpful for example if you use other browsers alongside Chrome, which don't prevent your from using out-of-date plugins.