In-place trimming of descriptor array when appending callbacks.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 92c2bc509ae7ff1b61d414841a9690c4361bf3cb..3694955eaf786bc04c07cdf400a4cb06be15ba3d 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -2635,6 +2635,10 @@ class DescriptorArray: public FixedArray {
   // fit in a page).
   static const int kMaxNumberOfDescriptors = 1024 + 512;
 
+  static int SizeFor(int number_of_descriptors) {
+    return ToKeyIndex(number_of_descriptors);
+  }
+
  private:
   // An entry in a DescriptorArray, represented as an (array, index) pair.
   class Entry {
@@ -4961,6 +4965,11 @@ class Map: public HeapObject {
                               Handle<Code> code);
   MUST_USE_RESULT MaybeObject* UpdateCodeCache(String* name, Code* code);
 
+  // Extend the descriptor array of the map with the list of descriptors.
+  // In case of duplicates, the latest descriptor is used.
+  static void CopyAppendCallbackDescriptors(Handle<Map> map,
+                                            Handle<Object> descriptors);
+
   // Returns the found code or undefined if absent.
   Object* FindInCodeCache(String* name, Code::Flags flags);