I did some searching, and what I found seemed very similar -- that there is a large amount of the Mac codebase that's in C++ and Carbon, and the UI segment is written in Cocoa. So it's not a pure Cocoa app. I didn't find any detailed technical analyses of the Mac version. You probably would have heard about it if there was a subsequent re-write of the C++ segment to make all of Chrome:Mac Cocoa.