Bug 1216040 - Make ArrayLength support mozilla::EnumeratedArray. r=froydnj

mfbt/ArrayUtils.h

@@ -20,6 +20,7 @@
 
 #include "mozilla/Alignment.h"
 #include "mozilla/Array.h"
+#include "mozilla/EnumeratedArray.h"
 #include "mozilla/TypeTraits.h"
 
 namespace mozilla {
@@ -64,6 +65,13 @@ ArrayLength(const Array<T, N>& aArr)
   return N;
 }
 
+template<typename E, E N, typename T>
+MOZ_CONSTEXPR size_t
+ArrayLength(const EnumeratedArray<E, N, T>& aArr)
+{
+  return size_t(N);
+}
+
 /*
  * Compute the address one past the last element of a constant-length array.
  *