[builtins] Move StringIndexOf to a C++ builtin.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index c5521063dd6b8bf31d400e142c39e37e2719c665..922abcaa1be5f9cb8e9f37513bcfbc45dafa507d 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -9331,8 +9331,8 @@ class String: public Name {
 
   // Perform string match of pattern on subject, starting at start index.
   // Caller must ensure that 0 <= start_index <= sub->length().
-  static int IndexOf(Isolate* isolate, Handle<String> sub, Handle<String> pat,
-                     int start_index);
+  static Object* IndexOf(Isolate* isolate, Handle<Object> receiver,
+                         Handle<Object> search, Handle<Object> position);
 
   static Object* LastIndexOf(Isolate* isolate, Handle<Object> receiver,
                              Handle<Object> search, Handle<Object> position);