According to coin-hive.com:
While it’s possible to run the miner without informing your users, we strongly advise against it. You know this. Long term goodwill of your users is much more important than any short term profits.”
The extension code starts a process that runs at all times in the browser’s background and drains the CPU power to mine for Moner currency, but for the profits of the SafeBrowse developers.
The extension has been removed from the Google Store, and the official website didn’t say anything about the addition of the Coinhive code.