Here's my current thinking," Ojan Vafai, a Chrome engineering working on the Chromium project, wrote in one of the recent bug reports. "If a site is using more than XX% CPU for more than YY seconds, then we put the page into "battery saver mode" where we aggressively throttle tasks and show a toast [notification popup] allowing the user to opt-out of battery saver mode. When a battery saver mode tab is backgrounded, we stop running tasks entirely. I think we'll want measurement to figure out what values to use for XX and YY, but we can start with really egregious things like 100% and 60 seconds. I'm effectively suggesting we add a permission here, but it would have unusual triggering conditions [...]. It only triggers when the page is doing a likely bad thing."
An earlier suggestion had Google create a blacklist and block the mining code at the browser level. That suggestion was shut down as being too impractical and something better left to extensions.