Ensure that 'once' objects are declared using the macro.  This is either the third or fourth time I've screwed this up when down-integrating, because our internal code does not require the macro (it's not portable) and on Linux a pthread_once_t that is zero-initialized just happens to work.  So, I only discover the problem when I test on Mac, then kick myself for making the same mistake yet again.  No more!  This time, I have renamed GoogleOnceType to ProtobufOnceType, thus making the type name differ from our internal code.  As a result, if you don't update the decls to use the macros, they won't compile.  Hah!  Take that, future self!