You may have stumbled upon a new extension called Google Cast API¹ in chrome://extensions and think: "What is it? I didn't install anything Chromecast related".
This extension is actually a Shared Module - a new kind of chrome extension - that can be used by other extensions and apps. On my Chromebook for instance, the Google Play Movies App and the Video Player App depend on it.

The only purpose of the Google Cast API is to indicate to the caller the most stable Google Cast Extension installed on the system since there may be² several³.

¹ https://chrome.google.com/webstore/detail/mafeflapfdfljijmlienjedomfjfmhpd ² https://chrome.google.com/webstore/detail/boadgeojelhgndaghljhdicfkmllpafd ³ https://chrome.google.com/webstore/detail/dliochdbjfkdbacpmhlcpmleaejidimm

Source: https://codereview.chromium.org/214693007
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