Fix: make string indexing work with Infinity.

src/x64/codegen-x64.cc

Index: src/x64/codegen-x64.cc
diff --git a/src/x64/codegen-x64.cc b/src/x64/codegen-x64.cc
index 1f3a5597ea82cde7a1f2c7353afb237e5353383d..d34543fcb362926aa4c1916fdae80080c8cfe255 100644
--- a/src/x64/codegen-x64.cc
+++ b/src/x64/codegen-x64.cc
@@ -10935,11 +10935,7 @@ void StringCharCodeAtGenerator::GenerateSlow(
   __ push(result_);
   __ push(index_);  // Consumed by runtime conversion function.
   if (index_flags_ == STRING_INDEX_IS_NUMBER) {
-    // Strictly speaking, NumberToInteger should be called here, but
-    // our string lengths don't exceed 32 bits and using ToUint32 maps
-    // -0 to 0, which is what is required by the spec when accessing
-    // strings.
-    __ CallRuntime(Runtime::kNumberToJSUint32, 1);
+    __ CallRuntime(Runtime::kNumberToIntegerMapMinusZero, 1);
   } else {
     ASSERT(index_flags_ == STRING_INDEX_IS_ARRAY_INDEX);
     // NumberToSmi discards numbers that are not exact integers.