Only use FreeSpace objects in the free list.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 51755054cb00a3513faae0b6c5684ce5473f105e..80ab91956b647041532acb280d5e611443c5a303 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -4448,8 +4448,11 @@ class ByteArray: public FixedArrayBase {
 };
 
 
-// FreeSpace represents fixed sized areas of the heap that are not currently in
-// use.  Used by the heap and GC.
+// FreeSpace are fixed-size free memory blocks used by the heap and GC.
+// They look like heap objects (are heap object tagged and have a map) so that
+// the heap remains iterable.  They have a size and a next pointer.
+// The next pointer is the raw address of the next FreeSpace object (or NULL)
+// in the free list.
 class FreeSpace: public HeapObject {
  public:
   // [size]: size of the free space including the header.
@@ -4461,7 +4464,12 @@ class FreeSpace: public HeapObject {
 
   inline int Size() { return size(); }
 
-  DECLARE_CAST(FreeSpace)
+  // Accessors for the next field.
+  inline FreeSpace* next();
+  inline FreeSpace** next_address();
+  inline void set_next(FreeSpace* next);
+
+  inline static FreeSpace* cast(HeapObject* obj);
 
   // Dispatched behavior.
   DECLARE_PRINTER(FreeSpace)
@@ -4470,9 +4478,7 @@ class FreeSpace: public HeapObject {
   // Layout description.
   // Size is smi tagged when it is stored.
   static const int kSizeOffset = HeapObject::kHeaderSize;
-  static const int kHeaderSize = kSizeOffset + kPointerSize;
-
-  static const int kAlignedSize = OBJECT_POINTER_ALIGN(kHeaderSize);
+  static const int kNextOffset = POINTER_SIZE_ALIGN(kSizeOffset + kPointerSize);
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(FreeSpace);