[runtime/heap] Introduce CopyFixedArrayUpTo to match CopyFixedArrayAndGrow, copying to a smaller ar…

src/builtins.cc

Index: src/builtins.cc
diff --git a/src/builtins.cc b/src/builtins.cc
index c6368bbd573c674417b4da5b37955b8b455f1e5e..29bfa2f00ef580258c88d764d2a01d63986b2b04 100644
--- a/src/builtins.cc
+++ b/src/builtins.cc
@@ -1869,10 +1869,7 @@ BUILTIN(ObjectKeys) {
       if (enum_length != 0) {
         Handle<FixedArray> cache(
             js_object->map()->instance_descriptors()->GetEnumCache());
-        keys = isolate->factory()->NewFixedArray(enum_length);
-        for (int i = 0; i < enum_length; i++) {
-          keys->set(i, cache->get(i));
-        }
+        keys = isolate->factory()->CopyFixedArrayUpTo(cache, enum_length);
       }
     }
   }