Make instance_size immediately useful for all fixed size objects.

test/cctest/test-heap.cc

Index: test/cctest/test-heap.cc
diff --git a/test/cctest/test-heap.cc b/test/cctest/test-heap.cc
index 9d5e1f19f9b467a4ec4d248fb3fea41e2462f8f6..605d8831307c20d3d63a74e7c1433e0f6e9a9a04 100644
--- a/test/cctest/test-heap.cc
+++ b/test/cctest/test-heap.cc
@@ -36,8 +36,8 @@ TEST(HeapMaps) {
   InitializeVM();
   CheckMap(Heap::meta_map(), MAP_TYPE, Map::kSize);
   CheckMap(Heap::heap_number_map(), HEAP_NUMBER_TYPE, HeapNumber::kSize);
-  CheckMap(Heap::fixed_array_map(), FIXED_ARRAY_TYPE, FixedArray::kHeaderSize);
-  CheckMap(Heap::string_map(), STRING_TYPE, SeqTwoByteString::kAlignedSize);
+  CheckMap(Heap::fixed_array_map(), FIXED_ARRAY_TYPE, kVariableSizeSentinel);
+  CheckMap(Heap::string_map(), STRING_TYPE, kVariableSizeSentinel);
 }