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

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index 2a54062f0a00019247f92d25b25ff85326e4e764..bac224f4e9fe044813e081dbba0592fe4a1e3ba6 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -2109,7 +2109,16 @@ void ExternalFloatArray::set(int index, float value) {
 }
 
 
-INT_ACCESSORS(Map, visitor_id, kScavengerCallbackOffset)
+int Map::visitor_id() {
+  return READ_BYTE_FIELD(this, kVisitorIdOffset);
+}
+
+
+void Map::set_visitor_id(int id) {
+  ASSERT(0 <= id && id < 256);
+  WRITE_BYTE_FIELD(this, kVisitorIdOffset, static_cast<byte>(id));
+}
+
 
 int Map::instance_size() {
   return READ_BYTE_FIELD(this, kInstanceSizeOffset) << kPointerSizeLog2;