Use ICU case conversion/transliterator for case conversion behind a flag

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 5b82ff5dcc42e049fcd42e242c2af55969524e7b..253685bf3a8352fb7fe760da08cebd890fd89731 100644
--- a/src/runtime/runtime-strings.cc
+++ b/src/runtime/runtime-strings.cc
@@ -1080,7 +1080,7 @@ MUST_USE_RESULT static Object* ConvertCase(
 
 RUNTIME_FUNCTION(Runtime_StringToLowerCase) {
   HandleScope scope(isolate);
-  DCHECK(args.length() == 1);
+  DCHECK_EQ(args.length(), 1);
   CONVERT_ARG_HANDLE_CHECKED(String, s, 0);
   return ConvertCase(s, isolate, isolate->runtime_state()->to_lower_mapping());
 }
@@ -1088,7 +1088,7 @@ RUNTIME_FUNCTION(Runtime_StringToLowerCase) {
 
 RUNTIME_FUNCTION(Runtime_StringToUpperCase) {
   HandleScope scope(isolate);
-  DCHECK(args.length() == 1);
+  DCHECK_EQ(args.length(), 1);
   CONVERT_ARG_HANDLE_CHECKED(String, s, 0);
   return ConvertCase(s, isolate, isolate->runtime_state()->to_upper_mapping());
 }