Google is rolling out an update to Chrome 51 (the current version) that will allow users to Cast without having to install the Google Cast extension. The update will add native Cast functionality to the browser and put a button in the tool menu. Users will also be able to right click on a page and click Cast, which will be faster for just about everyone.
The new Cast feature will remove the ability to determine video quality, bitrate, and resolution — Google says it will automatically adjust those settings dependent on your bandwidth. If you're currently using the Cast extension you don't have to change anything, there will just be an additional way to activate Google Cast. The update should be rolling out over the coming weeks.