Wait for sweeper threads in incremental marking step when sweeper threads are done sweeping.

src/sweeper-thread.h

 // Copyright 2012 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_SWEEPER_THREAD_H_
 #define V8_SWEEPER_THREAD_H_
 
 #include "atomicops.h"
 #include "flags.h"
 #include "platform.h"
 #include "utils.h"
 
 #include "spaces.h"
 
 #include "heap.h"
 
 namespace v8 {
 namespace internal {
 
 class SweeperThread : public Thread {
  public:
   explicit SweeperThread(Isolate* isolate);
   ~SweeperThread() {}
 
   void Run();
   void Stop();
   void StartSweeping();
   void WaitForSweeperThread();
+  bool SweepingCompleted();
 
   static int NumberOfThreads(int max_available);
 
  private:
   Isolate* isolate_;
   Heap* heap_;
   MarkCompactCollector* collector_;
   Semaphore start_sweeping_semaphore_;
   Semaphore end_sweeping_semaphore_;
   Semaphore stop_semaphore_;
   volatile AtomicWord stop_thread_;
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_SWEEPER_THREAD_H_