Ensure that 'once' objects are declared using the macro. This is either the...

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!