Increment descriptor array slack for prototypes by a constant rather than 50%

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index a517615d4b8f0c669172b834caa65f003b6c4b29..bed2d0af335247cb94111a633083333b3430d201 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -6784,8 +6784,9 @@ Handle<Map> Map::ShareDescriptor(Handle<Map> map,
     if (old_size == 0) {
       descriptors = DescriptorArray::Allocate(map->GetIsolate(), 0, 1);
     } else {
-      EnsureDescriptorSlack(
-          map, SlackForArraySize(old_size, kMaxNumberOfDescriptors));
+      int slack = SlackForArraySize(map->is_prototype_map(), old_size,
+                                    kMaxNumberOfDescriptors);
+      EnsureDescriptorSlack(map, slack);
       descriptors = handle(map->instance_descriptors());
     }
   }