According to the chrome feature status page chrome should have this enabled by default starting version 60 but SharedArrayBuffer and Atomics are undefined on the following:
Instead on all of these there is a flag in chrome://flags to enable them. Note that node.js 8.x does have them enabled by default. Why do I not have these available?
If you follow that status link and go to its documentation line you end up on MDN: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/SharedArrayBuffer
Under compatibility you'll note Chrome gets as asterisk with the note:
Chrome disabled SharedArrayBuffer on January 5, 2018 to help reduce the efficacy of speculative side-channel attacks. This is intended as a temporary measure until other mitigations are in place.