[heap] Start concurrent marking simultaneously with incremental marking.

src/heap/concurrent-marking.h

 // Copyright 2017 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_HEAP_CONCURRENT_MARKING_
 #define V8_HEAP_CONCURRENT_MARKING_
 
 #include <vector>
 
 #include "src/allocation.h"
 #include "src/cancelable-task.h"
 #include "src/utils.h"
 #include "src/v8.h"
 
 namespace v8 {
 namespace internal {
 
 class Heap;
 class Isolate;
 
 class ConcurrentMarking {
  public:
   explicit ConcurrentMarking(Heap* heap);
   ~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_;
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_HEAP_PAGE_PARALLEL_JOB_