Since Google Chrome's version 51, Chromecast functionality is available in the Chrome Tools Menu
click the Chrome  in the top right corner of your Chrome browser window. Then click Cast…:

Cast from Chrome

Chromecast functionality is also available when right clicking on a webpage in the Chrome browser (Control+click on Mac.)

Note: To access Chromecast functionality via the the Cast button in your Chrome toolbar, you must pin the Cast button to your Chrome toolbar.