Turn visitor_id into a byte field and free back one word in Map.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 11d65ef44b6c9265c5b3a7c245e90d84520fcbd8..ca668b4d2d5bfa10b77fac12ed6a977d253f3ad5 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -3245,8 +3245,7 @@ class Map: public HeapObject {
   static const int kInstanceDescriptorsOffset =
       kConstructorOffset + kPointerSize;
   static const int kCodeCacheOffset = kInstanceDescriptorsOffset + kPointerSize;
-  static const int kScavengerCallbackOffset = kCodeCacheOffset + kPointerSize;
-  static const int kPadStart = kScavengerCallbackOffset + kPointerSize;
+  static const int kPadStart = kCodeCacheOffset + kPointerSize;
   static const int kSize = MAP_POINTER_ALIGN(kPadStart);
 
   // Layout of pointer fields. Heap iteration code relies on them
@@ -3263,9 +3262,8 @@ class Map: public HeapObject {
   static const int kPreAllocatedPropertyFieldsByte = 2;
   static const int kPreAllocatedPropertyFieldsOffset =
       kInstanceSizesOffset + kPreAllocatedPropertyFieldsByte;
-  // The byte at position 3 is not in use at the moment.
-  static const int kUnusedByte = 3;
-  static const int kUnusedOffset = kInstanceSizesOffset + kUnusedByte;
+  static const int kVisitorIdByte = 3;
+  static const int kVisitorIdOffset = kInstanceSizesOffset + kVisitorIdByte;
 
   // Byte offsets within kInstanceAttributesOffset attributes.
   static const int kInstanceTypeOffset = kInstanceAttributesOffset + 0;