From https://chromium-review.googlesource.com/#/c/563157/ :

"""We don't appear to use a lot of builtin function definitions (the only one I can find in "Chromium Framework" is __umodti3), but it appears that /usr/lib/system/libcompiler_rt.dylib (which is part of the SDK) provides definitions of the builtin functions that existed when the SDK was built, so we end up referencing that. In this case, the _IsOSVersionAtLeast function is relatively new, so it won't be present in the SDK's libcompiler_rt.dylib.

Somewhat surprisingly, the linker seems to prefer definitions in the .dylib to definitions in the .a. A consequence of that is that it doesn't make sense to me how we can correctly target 10.9 with a 10.10 SDK if the SDK's .dylib is from 10.10. Couldn't that result in undefined references at runtime if the executable ends up referring to symbols in the .dylib that were added in 10.10? Maybe Apple has some mechanism to deal with that, I'm not sure."""

That sounds like something we should understand.

Since our 10.10 SDK builds currently do run on 10.9 somehow, things seem to work out.