In-place shrinking of descriptor arrays with non-live transitions.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 692df446e262460e0f52f4dc899612f1d43451ad..23b3306ce61da953c589a319dfb37a1f3d4bb695 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -2501,9 +2501,13 @@ class DescriptorArray: public FixedArray {
   // Accessors for fetching instance descriptor at descriptor number.
   inline String* GetKey(int descriptor_number);
   inline Object** GetKeySlot(int descriptor_number);
+  inline void SetKeyUnchecked(Heap* heap, int descriptor_number, String* value);
   inline Object* GetValue(int descriptor_number);
   inline Object** GetValueSlot(int descriptor_number);
-  inline void SetNullValueUnchecked(int descriptor_number, Heap* heap);
+  inline void SetNullValueUnchecked(Heap* heap, int descriptor_number);
+  inline void SetValueUnchecked(Heap* heap,
+                                int descriptor_number,
+                                Object* value);
   inline PropertyDetails GetDetails(int descriptor_number);
   inline void SetDetailsUnchecked(int descriptor_number, Smi* value);
   inline PropertyType GetType(int descriptor_number);
@@ -2513,7 +2517,6 @@ class DescriptorArray: public FixedArray {
   inline AccessorDescriptor* GetCallbacks(int descriptor_number);
   inline bool IsProperty(int descriptor_number);
   inline bool IsTransitionOnly(int descriptor_number);
-  inline bool IsNullDescriptor(int descriptor_number);
 
   // WhitenessWitness is used to prove that a specific descriptor array is white
   // (unmarked), so incremental write barriers can be skipped because the
@@ -4612,6 +4615,7 @@ class Map: public HeapObject {
   // TODO(1399): It should be possible to make room for bit_field3 in the map
   // without overloading the instance descriptors field (and storing it in the
   // DescriptorArray when the map has one).
+  inline void SetOwnBitField3(int value);
   inline int bit_field3();
   inline void set_bit_field3(int value);