Partially revert r29468

src/objects.cc

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <iomanip>
 #include <sstream>
 
 #include "src/v8.h"
 
 #include "src/accessors.h"
@@ skipping 6744 matching lines @@
 
   Handle<Map> result = CopyDropDescriptors(map);
   Handle<Name> name = descriptor->GetKey();
 
   // Ensure there's space for the new descriptor in the shared descriptor array.
   if (descriptors->NumberOfSlackDescriptors() == 0) {
     int old_size = descriptors->number_of_descriptors();
     if (old_size == 0) {
       descriptors = DescriptorArray::Allocate(map->GetIsolate(), 0, 1);
     } else {
-      int slack = SlackForArraySize(map->is_prototype_map(), old_size,
-                                    kMaxNumberOfDescriptors);
+      int slack = SlackForArraySize(old_size, kMaxNumberOfDescriptors);
       EnsureDescriptorSlack(map, slack);
       descriptors = handle(map->instance_descriptors());
     }
   }
 
   Handle<LayoutDescriptor> layout_descriptor =
       FLAG_unbox_double_fields
           ? LayoutDescriptor::ShareAppend(map, descriptor->GetDetails())
           : handle(LayoutDescriptor::FastPointerLayout(), map->GetIsolate());
 
@@ skipping 9350 matching lines @@
                                             Handle<Object> new_value) {
   if (cell->value() != *new_value) {
     cell->set_value(*new_value);
     Isolate* isolate = cell->GetIsolate();
     cell->dependent_code()->DeoptimizeDependentCodeGroup(
         isolate, DependentCode::kPropertyCellChangedGroup);
   }
 }
 }  // namespace internal
 }  // namespace v8