Make old-space heap stats safe for parallel marking.

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 31 matching lines @@
   void IterateWeakRoots(Isolate* isolate,
                         HandleVisitor* visitor,
                         bool visit_prologue_weak_persistent_handles);
   void IterateWeakReferences(Isolate* isolate, MarkingVisitor* visitor);
   void ProcessWeakTables(PageSpace* page_space);
   void ProcessObjectIdTable(Isolate* isolate);
 
   // Called by anyone: finalize and accumulate stats from 'visitor'.
   void FinalizeResultsFrom(MarkingVisitor* visitor);
 
-  Monitor monitor_;  // Protects marked_bytes_ and done_count_.
   Heap* heap_;
+
+  Mutex stats_mutex_;
+  // TODO(koda): Remove after verifying it's redundant w.r.t. ClassHeapStats.
   uintptr_t marked_bytes_;
 
   friend class MarkTask;
   DISALLOW_IMPLICIT_CONSTRUCTORS(GCMarker);
 };
 
 }  // namespace dart
 
 #endif  // VM_GC_MARKER_H_