`mozilla::StringBuffer` uses `RefCountLogger`, but this was a no-op when included in SpiderMonkey code (`MOZILLA_INTERNAL_API` is not defined in that case). This resulted in leakcheck failures when string buffers are used directly in SpiderMonkey. This patch changes the calls to `NS_LogAddRef` and `NS_LogRelease` to go through a function pointer. This also makes it possible to use a different implementation in SpiderMonkey shell builds if we ever want to. Differential Revision: https://phabricator.services.mozilla.com/D213968