Google is rolling out some changes to Casting in the desktop version Chrome with the latest release, 51. While you could previously cast from the browser using the Google Cast toolbar icon, this will no longer be necessary going forward.

Chrome builds in direct Google Cast support, no extension required

From Google:

Google Cast functionality is now built in as part of the Chrome browser itself, so you no longer need to install the Google Cast toolbar icon in order to use your Google Cast device from Chrome. 

If you previously installed the Google Cast toolbar icon, there's no need to remove it.  The toolbar icon will continue to provide quick access to Google Cast functionality.

Google Cast functionality will be available through Chrome's Tools menu in the upper right corner of the browser window, in the same section as functions like Print and Find. Additionally, you can right-click a page's tab to get an option to cast.

Chrome 51 also lets you cast to a Hangout. This allows you to mirror the contents of any tab to a Hangout.

This new functionality is rolling out to Chrome for desktops now, though it could be a little while before you can use it yourself.