My guess is there isn't an enforced MySQL version across all of Google.
"We asked Google for more information, and the company sent us a statement which said: "Google's MySQL team is in the process of moving internal users of MySQL at Google from MySQL 5.1 to MariaDB 10.0. Google's MySQL team and the SkySQL MariaDB team are looking forward to working together to advance the reliability and feature set of MariaDB."
"Q: Why didn't you base this on MariaDB, Percona Server, Drizzle, etc....
A: We reached a consensus that MySQL-5.6 was the right choice for this, as it has the production-ready features we need to operate at scale, and the features planned for MySQL-5.7 seem like a fitting path forward for us. We will continue to revisit this decision as the ecosystem evolves."
MariaDB 10 just hit after the launch, much less conception, of Webscale, and still doesn't have all of the features from MySQL 5.6 in it.