Google recently added a 'global media controls' flag to Chrome that lets you control foreground and background media right from the browser's toolbar. The feature has since arrived on the stable release of Chrome, though it still needs to be activated via chrome://flags/#global-media-controls before you can use it. To make it even handier, Google seems to be working on adding the ability to control cast content via these playback buttons.
An entry in the Chromium Gerrit is pretty straight forward in showing what Google is developing: "Add a flag to show Cast sessions in the Global Media Controls UI." Further comments in the corresponding Chromium Bug Tracker thread indicate that the media buttons will automatically appear when there is an active Cast session. That would bring Chrome on par with Android, which has been providing you with Cast playback controls for a long time.
The regular playback control buttons for media playing in the browser.
Presumably, the Cast controls will look just like regular playback buttons. Depending on how well a website or service supports the standard, you'll get pause, skipping, and scrubbing options along with the media's cover or poster. This will certainly come in handy when you're sitting on the couch surfing the web on Chrome while watching cast content on your TV.
- Chrome Story