[builtins] Migrate String.prototype.charCodeAt and String.prototype.charAt to TurboFan.

src/runtime/runtime-strings.cc

Index: src/runtime/runtime-strings.cc
diff --git a/src/runtime/runtime-strings.cc b/src/runtime/runtime-strings.cc
index 6786fa99fba0f92e1c41e5e228d9f6852349dd74..5b82ff5dcc42e049fcd42e242c2af55969524e7b 100644
--- a/src/runtime/runtime-strings.cc
+++ b/src/runtime/runtime-strings.cc
@@ -1273,6 +1273,13 @@ RUNTIME_FUNCTION(Runtime_StringCharAt) {
   return __RT_impl_Runtime_StringCharFromCode(Arguments(1, &code), isolate);
 }
 
+RUNTIME_FUNCTION(Runtime_ExternalStringGetChar) {
+  SealHandleScope shs(isolate);
+  DCHECK_EQ(2, args.length());
+  CONVERT_ARG_CHECKED(ExternalString, string, 0);
+  CONVERT_INT32_ARG_CHECKED(index, 1);
+  return Smi::FromInt(string->Get(index));
+}
 
 RUNTIME_FUNCTION(Runtime_OneByteSeqStringGetChar) {
   SealHandleScope shs(isolate);