Doom 3 actually used Scons across all the OSes (~2004). At the time, it was so nice to have a python build system. I sort of hoped it was the future, but it sort of died as it failed to scale. I've seen a few home-brewed python build systems work well, but typically we're back to CMake/Make
I would expect kernels to be quite small (#file, line-count) compared to major Applications like Unreal Games and Civilization 5. I've never worked on Chrome, but I can safely say the amount of source code in a few Unreal Games and Civilization5 dwarfs the drivers and OS code I've worked on. Take Unreal then add a team of developers adding onto it for multiple years thru multiple releases. Then add all the middleware (Havok, AudioEngines, NaturalMotion).
OS are much larger than kernel, I'd guess all the driver code exceeds the actual kernel.
People always think they code base is large, but having built most of the Call of Duties and many Unreal games, all the OS code I've worked on is trivial in size comparison. There is probably something bigger, but games seem bigger than many major apps in my experience.
Not to take away from your point, but in my experience the vast majority of C++ build pipelines even at major companies can still be improved. Few people enjoy 'improving the build', it often touches everything, and requires discipline to keep it working. Most of the projects I've worked on have been larger than Chrome, I've seen the compile time for BioShock Infinite go from 2 hours down to 15 minutes with serious work on header use, precompiled headers, and all the other tricks people use. Epic's build system is a pretty good example. There is even a older book, Large-scale C++ Design, that is specifically about this point.
Starting with a full build that initially takes hours and it shrinking to < 15-20 minutes and better seems pretty par for the course for truely large C++ projects. You don't get a fast build process for free, but if the team makes it a priority, alot can be done.
EDIT: Times mentioned were for a full build, often you rarely due a full build, incremental builds should be majority. Places that don't make incremental builds 100% reliable drive me crazy and waste so much developer time. This is common, but it's a lame excuse. Just do the work and fix it.
EDIT: I'm sure lots of work has been done, not trying to degrade that. Just sharing my experience on my projects, never worked with Chrome.