Decoupling MarkDescriptorArray as much as possible from the ContentArray.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index f5b8ab67f5204a1e033d7ac58121e2ba812ded5e..076368830a934e3fa16080923443d16f75e1c225 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -2453,6 +2453,7 @@ class DescriptorArray: public FixedArray {
   // Accessors for fetching instance descriptor at descriptor number.
   inline String* GetKey(int descriptor_number);
   inline Object* GetValue(int descriptor_number);
+  inline Object** GetValueSlot(int descriptor_number);
   inline PropertyDetails GetDetails(int descriptor_number);
   inline PropertyType GetType(int descriptor_number);
   inline int GetFieldIndex(int descriptor_number);
@@ -2463,6 +2464,14 @@ class DescriptorArray: public FixedArray {
   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
+  // marking invariant cannot be broken and slots pointing into evacuation
+  // candidates will be discovered when the object is scanned. A witness is
+  // always stack-allocated right after creating a descriptor array. By
+  // allocating a witness, incremental marking is globally disabled. The witness
+  // is then passed along wherever needed to statically prove that the
+  // descriptor array is known to be white.
   class WhitenessWitness {
    public:
     inline explicit WhitenessWitness(DescriptorArray* array);
@@ -2591,6 +2600,8 @@ class DescriptorArray: public FixedArray {
   static const int kMaxNumberOfDescriptors = 1024 + 512;
 
  private:
+  friend class IntrusiveMapTransitionIterator;
+
   // An entry in a DescriptorArray, represented as an (array, index) pair.
   class Entry {
    public:
@@ -2629,6 +2640,7 @@ class DescriptorArray: public FixedArray {
   FixedArray* GetContentArray() {
     return FixedArray::cast(get(kContentArrayIndex));
   }
+
   DISALLOW_IMPLICIT_CONSTRUCTORS(DescriptorArray);
 };