There are only 13 questions using this tag, so it should be easy to split.
Done. Since there were only 13 questions affected and a decision had to be made about which native client they were talking about, I went ahead and did this myself.
There are still some of the Google questions that use both google-nativeclient and google-nacl, and some that have only one tag without the other. I left those alone, instead supporting this feature request since that will be automatically taken care of when the tag synonym is created by a moderator. There's no risk of ambiguity there: all questions with either or both of those tags are clearly talking about Google's Native Client.