a) Government A decides target B has valuable communications, and uses this audio capture functionality as an attack vector (ie, a MiTM server modifies the chrome binary blob request slightly to a version where chunked audio is sent back to a control server).
b) (more likely) This binary blob contains a voice recognition algorithm, which can of course detect the phrase "ok, google." Imagine they wanted to detect other phrases, like "drugs" or "travel." Small modifications could easily allow an arbitrary list of "hot" terms to be targeted. Then no audio is even sent back from the user's computer--a small flag in your google account is simply set attaching your profile to "high risk" terms overheard, and databased, where it could later be queried by law enforcement.
It's troubling because there's no transparency, and if you spend a little time brainstorming about the ways this could be used maliciously (most likely by a nation-state) there are many possibilities...