Currently, GuestView callers are #ifdef'd behind ENABLE_EXTENSIONS and sometimes !OS_ANDROID. Although nothing outside of extensions currently uses GuestViews, it would be better to have GuestViews decoupled from extensions for clarity, consistency and to allow for potential GuestView usage on mobile (see also  crbug.com/444869  ).

We should add an ENABLE_GUESTVIEW build flag which can, for now, just be an alias for ENABLE_EXTENSIONS, and then update the caller #ifdefs and build files to use it.