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 4fd29ad56227d368d2ebb0773c566f6cba3909b0..ba8b32d6eee99c5bdb4dce1a5303b37a07da7ab9 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -2474,7 +2474,12 @@ 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 Object* RawGetValue(int descriptor_number);
+  inline PropertyDetails RawGetDetails(int descriptor_number);
+
   inline PropertyType GetType(int descriptor_number);
   inline int GetFieldIndex(int descriptor_number);
   inline JSFunction* GetConstantFunction(int descriptor_number);
@@ -2484,6 +2489,14 @@ 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);
@@ -2650,6 +2663,7 @@ class DescriptorArray: public FixedArray {
   FixedArray* GetContentArray() {
     return FixedArray::cast(get(kContentArrayIndex));
   }
+  inline FixedArray* RawGetContentArray();
   DISALLOW_IMPLICIT_CONSTRUCTORS(DescriptorArray);
 };