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

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 02e3b4f89d3ba4aed5ca912278ca5d1cd2c1063c..6f96a69d031e48ba296bf5aa654b3a625432ab8d 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -8861,6 +8861,12 @@ class String: public Name {
   // Conversion.
   inline bool AsArrayIndex(uint32_t* index);
 
+  // Case conversion.
+  static MaybeHandle<String> ToLowerCase(Handle<String> string)
+      WARN_UNUSED_RESULT;
+  static MaybeHandle<String> ToUpperCase(Handle<String> string)
+      WARN_UNUSED_RESULT;
+
   // Trimming.
   enum TrimMode { kTrim, kTrimLeft, kTrimRight };
   static Handle<String> Trim(Handle<String> string, TrimMode mode);