Race is self-reported. There isn't someone in Google HR researching your ancestry, they ask you what race you identify as when you join. If you self-identify as white then you are white according to Google HR.
Interesting, that introduces some very skewed incentives: from a game theory perspective it would always be better to lie about your race in HR surveys. Then according to the data, your actual race would be underrepresented and the industry would try to hire more people who look like you, making your next job search easier.