Up until now, the Google Cast extension was needed in Chrome to send content between the device and a Chromecast. But now with Chrome 51, Google has integrated Cast functionality directly into the browser itself. Google says that if you have the Google Cast toolbar icon extension, there is no need to remove it because it"ll still offer quick access to the Google Cast functionality.
Casting between the browser and a Chromecast connected device works by allowing users to stream any page to the Chromecast connected device (ie HDMI TV). To access Cast without the extension installed users need to head over to the Chrome Settings menu and go to the Cast option which is found between Print and Find midway down the menu. Alternatively, users can right-click the tab that they want to cast and then select Cast from the pop-up menu.
As well as integrating the Casting feature, Google has simplified the process. In earlier versions of the Cast toolbar icon extension, you had the option to set the resolution, bitrate, quality, and other features when mirroring the contents of a tab to the Cast device. Going forward, these options have been removed, instead, the system will now automatically adjust the settings based on the quality of the network.
The last major change to Google Cast in Chrome 51 is the ability to cast tabs into Google Hangouts.
Google says it is rolling out the functionality. The rollout appears to be staggered as not everyone can see the Cast option in the menu just yet.
Source: Google