Add last OS error into heap stats.

src/heap.h

Index: src/heap.h
diff --git a/src/heap.h b/src/heap.h
index ac00827d3834717bed0563e242668dd8057a61fe..93b90b1846acbab3717e0ffc61c195dc06079bba 100644
--- a/src/heap.h
+++ b/src/heap.h
@@ -1345,7 +1345,8 @@ class HeapStats {
   int* memory_allocator_capacity;       // 20
   int* objects_per_type;                // 21
   int* size_per_type;                   // 22
-  int* end_marker;                      // 23
+  int* os_error;                        // 23
+  int* end_marker;                      // 24
 };