Engineers capable of working at Google != majoring in computer science.
Computer science degrees are handed out like cracker jack prizes these days.
Men graduate college at a lower rate than women, graduate high school at a lower rate, and have a lower literacy rate. What does that have to do with the specific pool of talent from which an uber engineering company like Google would pull from?
17% of "tech" employees at Google are women (worldwide): http://www.google.com/diversity/at-google.html#tab=tech
18% of CS degrees are earned by women (in the US): http://nces.ed.gov/programs/digest/d12/tables/dt12_349.asp
so women are not in in fact over-represented among Google engineers, at least by that measure.