Splitting DescriptorArray::CopyInsert into CopyInsert and CopyReplace.

src/runtime.cc

Index: src/runtime.cc
diff --git a/src/runtime.cc b/src/runtime.cc
index 42d19aacde1999e01c454c35039122ff95743d37..a64a97f3afaf30a7a3b42cc2cf84e2bac3d17d4b 100644
--- a/src/runtime.cc
+++ b/src/runtime.cc
@@ -2181,7 +2181,8 @@ RUNTIME_FUNCTION(MaybeObject*, Runtime_FunctionSetReadOnlyPrototype) {
         details.index());
     // Construct a new field descriptors array containing the new descriptor.
     DescriptorArray* new_descriptors;
-    { MaybeObject* maybe_descriptors = instance_desc->CopyInsert(&new_desc);
+    { MaybeObject* maybe_descriptors =
+          instance_desc->CopyReplace(index, &new_desc);
       if (!maybe_descriptors->To(&new_descriptors)) return maybe_descriptors;
     }
     // Create a new map featuring the new field descriptors array.