[builtins] Move StringIndexOf to a builtin.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 5afcdc4736acde850ff7bfb95144a9c8beb874ac..235ed48b41ca09f8b15928c9d850c8c59ba17357 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -9248,8 +9248,8 @@ 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> pattern, Handle<Object> position);
 
   static Object* LastIndexOf(Isolate* isolate, Handle<Object> receiver,
                              Handle<Object> search, Handle<Object> position);