But not using rtti *within* webrtc (i.e., not using dynamic_cast or any other features requiring rtti info in the generated code) is a different issue than enabling building webrtc with rtti support, to help *other* applications which do use rtti, and also want to use webrtc.
I don't understand all the fine details of rtti in object files, but I think it's preferable to have all object files linked into an executable be compiled with the same rtti-related options, so it would make the most sense as a top-level build option.
And similar considerations apply to building with support for exceptions and stack unwind magic, I imagine.
/Niels