Bug 792689 - Add some double negations to the macros in Likely.h; r=Waldo

mfbt/Likely.h

@@ -12,11 +12,11 @@
 #define mozilla_Likely_h_
 
 #if defined(__clang__) || (defined(__GNUC__) && (__GNUC__ > 2))
-#  define MOZ_LIKELY(x)   (__builtin_expect((x), 1))
-#  define MOZ_UNLIKELY(x) (__builtin_expect((x), 0))
+#  define MOZ_LIKELY(x)   (__builtin_expect(!!(x), 1))
+#  define MOZ_UNLIKELY(x) (__builtin_expect(!!(x), 0))
 #else
-#  define MOZ_LIKELY(x)   (x)
-#  define MOZ_UNLIKELY(x) (x)
+#  define MOZ_LIKELY(x)   (!!(x))
+#  define MOZ_UNLIKELY(x) (!!(x))
 #endif
 
 #endif /* mozilla_Likely_h_ */