Refactor delay set to allow more than one marking visitor

runtime/vm/gc_marker.h

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_GC_MARKER_H_
 #define VM_GC_MARKER_H_
 
 #include "vm/allocation.h"
 
 namespace dart {
@@ skipping 33 matching lines @@
   void IterateWeakReferences(Isolate* isolate, MarkingVisitor* visitor);
   void DrainMarkingStack(Isolate* isolate, MarkingVisitor* visitor);
   void ProcessWeakProperty(RawWeakProperty* raw_weak, MarkingVisitor* visitor);
   void ProcessWeakTables(PageSpace* page_space);
   void ProcessObjectIdTable(Isolate* isolate);
 
 
   Heap* heap_;
   intptr_t marked_bytes_;
 
+  friend class DrainTask;
   DISALLOW_IMPLICIT_CONSTRUCTORS(GCMarker);
 };
 
 }  // namespace dart
 
 #endif  // VM_GC_MARKER_H_