[builtins] Also migrate String.prototype.toLowerCase/toUpperCase to C++.

src/builtins.cc

Index: src/builtins.cc
diff --git a/src/builtins.cc b/src/builtins.cc
index b2cf4861ec0ed6f7cb462e6173807f1b70d311cd..d5745773d897a17d39971b39b7db8e23a28a1eda 100644
--- a/src/builtins.cc
+++ b/src/builtins.cc
@@ -4554,6 +4554,24 @@ void Builtins::Generate_StringPrototypeCharCodeAt(
   assembler->Return(result);
 }
 
+// ES6 section 21.1.3.22 String.prototype.toLowerCase ( )
+BUILTIN(StringPrototypeToLowerCase) {
+  HandleScope scope(isolate);
+  TO_THIS_STRING(string, "String.prototype.toLowerCase");
+  ASSIGN_RETURN_FAILURE_ON_EXCEPTION(isolate, string,
+                                     String::ToLowerCase(string));
+  return *string;
+}
+
+// ES6 section 21.1.3.24 String.prototype.toUpperCase ( )
+BUILTIN(StringPrototypeToUpperCase) {
+  HandleScope scope(isolate);
+  TO_THIS_STRING(string, "String.prototype.toUpperCase");
+  ASSIGN_RETURN_FAILURE_ON_EXCEPTION(isolate, string,
+                                     String::ToUpperCase(string));
+  return *string;
+}
+
 // ES6 section 21.1.3.25 String.prototype.trim ()
 BUILTIN(StringPrototypeTrim) {
   HandleScope scope(isolate);