Google has begun rolling out a built-in Cast option for Chrome 51. You simply have to right click a tab or click the hamburger button on the right-hand part of the desktop browser to find a line in the menu that says "Cast..." Doesn't mean the old extension is completely useless now, though -- you don't need it to beam anything to your Chromecast anymore, but you could still use it as a shortcut. Besides, it looks like the feature isn't available for everyone just yet, even for those with up-to-date Chrome browsers. If you still don't have access to it, you may want to hold off on uninstalling the extension and checking your menus every now and then.