Bug 1606568 - Add JS::ConstLatin1Chars. r=jwalden

Useful for the public BigInt API which takes Range<const CharT>. Allows

JS::NumberToBigInt(JSContext*, JS::ConstLatin1Chars)

in order to match

JS::NumberToBigInt(JSContext*, JS::ConstTwoByteChars)

which already exists.

Differential Revision: https://phabricator.services.mozilla.com/D82797

js/public/CharacterEncoding.h

@@ -40,6 +40,20 @@ class Latin1Chars : public mozilla::Range<Latin1Char> {
              aLength) {}
 };
 
+/*
+ * Like Latin1Chars, but the chars are const.
+ */
+class ConstLatin1Chars : public mozilla::Range<const Latin1Char> {
+  typedef mozilla::Range<const Latin1Char> Base;
+
+ public:
+  using CharT = Latin1Char;
+
+  ConstLatin1Chars() = default;
+  ConstLatin1Chars(const Latin1Char* aChars, size_t aLength)
+      : Base(aChars, aLength) {}
+};
+
 /*
  * A Latin1Chars, but with \0 termination for C compatibility.
  */