I'm not sure what you mean by "optimized for use in browsers". I can't speak to the other 2, but Chromium is basically a vast collection of C++ cross-platform libraries that can be used in many applications.
While a class/library/etc might be used by chrome and all of its many users, it was originally designed to solve an issue for the browser and it might have a horrible api for anything else or much more likely be completely missing api that other applications would not only want, but need.
Actually if I'm not mistaken, the /base /net /threading (lower level components) etc. are common with the google server side codebase. At the very least, they share a common heritage. Whether they're kept in-sync on a regular basis... I couldn't say.
"optimized for use in browsers" means that collection of C++ cross-platform libraries can segfault a production server simply because it was not enough tested in other environments.