[heap] Start concurrent marking simultaneously with incremental marking.

src/heap/concurrent-marking.h

Index: src/heap/concurrent-marking.h
diff --git a/src/heap/concurrent-marking.h b/src/heap/concurrent-marking.h
index 70842cd230d01c13810026bd5c28f4c6a0aa2c1a..9d7b6b58caa2ceda29bd723e5f6cb41942f179f2 100644
--- a/src/heap/concurrent-marking.h
+++ b/src/heap/concurrent-marking.h
@@ -25,13 +25,16 @@ class ConcurrentMarking {
 
   void AddRoot(HeapObject* object);
 
-  void StartMarkingTask();
+  void StartTask();
   void WaitForTaskToComplete();
+  bool IsTaskPending() { return is_task_pending_; }
+  void EnsureTaskCompleted();
 
  private:
   class Task;
   Heap* heap_;
-  base::Semaphore pending_task_;
+  base::Semaphore pending_task_semaphore_;
+  bool is_task_pending_;
   std::vector<HeapObject*> root_set_;
 };