Bug 1124029 - Fix unsigned/signed mismatch in MathAlgorithms. r=waldo

--HG--
extra : rebase_source : d4ddeacc763ebe9266ad40d4c63f343033b16490
extra : source : 7ca6aa20529ebc304c14c1e21160fd65816415b3

mfbt/MathAlgorithms.h

@@ -382,7 +382,7 @@ public:
   static uint_fast8_t compute(const T aValue)
   {
     // Check for <= 1 to avoid the == 0 undefined case.
-    return aValue <= 1 ? 0 : 64 - CountLeadingZeroes64(aValue - 1);
+    return aValue <= 1 ? 0u : 64u - CountLeadingZeroes64(aValue - 1);
   }
 };