Use just one marking deque.

src/heap/mark-compact.h

Index: src/heap/mark-compact.h
diff --git a/src/heap/mark-compact.h b/src/heap/mark-compact.h
index b1ad5b8a80ee6547e9a03708a473a46c913822ce..c694d2988decfe6b3d8dc73c7542d2699713fd7e 100644
--- a/src/heap/mark-compact.h
+++ b/src/heap/mark-compact.h
@@ -655,6 +655,14 @@ class MarkCompactCollector {
   // to artificially keep AllocationSites alive for a time.
   void MarkAllocationSite(AllocationSite* site);
 
+  MarkingDeque* marking_deque() { return &marking_deque_; }
+
+  void EnsureMarkingDequeIsCommittedAndInitialize();
+
+  void InitializeMarkingDeque();
+
+  void UncommitMarkingDeque();
+
  private:
   class SweeperTask;
 
@@ -875,6 +883,8 @@ class MarkCompactCollector {
 #endif
 
   Heap* heap_;
+  base::VirtualMemory* marking_deque_memory_;
+  bool marking_deque_memory_committed_;
   MarkingDeque marking_deque_;
   CodeFlusher* code_flusher_;
   bool have_code_to_deoptimize_;