Bug 1106050 - Fix variable ‘fifoCallbacksRegistered’ set but not used warning. r=njn

xpcom/base/nsMemoryInfoDumper.cpp

@@ -7,6 +7,7 @@
 #include "mozilla/JSONWriter.h"
 #include "mozilla/UniquePtr.h"
 #include "mozilla/nsMemoryInfoDumper.h"
+#include "mozilla/DebugOnly.h"
 #include "nsDumpUtils.h"
 
 #include "mozilla/unused.h"
@@ -221,7 +222,7 @@ doGCCCDump(const nsCString& aInputStr)
 bool
 SetupFifo()
 {
-  static bool fifoCallbacksRegistered = false;
+  static DebugOnly<bool> fifoCallbacksRegistered = false;
 
   if (!FifoWatcher::MaybeCreate()) {
     return false;