Re-land "Concurrently unmap free pages."

src/heap/heap.h

Index: src/heap/heap.h
diff --git a/src/heap/heap.h b/src/heap/heap.h
index 15b047c62e553b83b7bed625b427724b4d383249..1889cede8b94833eaaed2939f7d91dda6dae6f7a 100644
--- a/src/heap/heap.h
+++ b/src/heap/heap.h
@@ -1423,7 +1423,9 @@ class Heap {
 
   void QueueMemoryChunkForFree(MemoryChunk* chunk);
   void FilterStoreBufferEntriesOnAboutToBeFreedPages();
+  void FreeQueuedChunks(MemoryChunk* list_head);
   void FreeQueuedChunks();
+  void WaitUntilUnmappingOfFreeChunksCompleted();
 
   int gc_count() const { return gc_count_; }
 
@@ -1600,6 +1602,8 @@ class Heap {
   bool ShouldOptimizeForMemoryUsage() { return optimize_for_memory_usage_; }
 
  private:
+  class UnmapFreeMemoryTask;
+
   static const int kInitialStringTableSize = 2048;
   static const int kInitialEvalCacheSize = 64;
   static const int kInitialNumberStringCacheSize = 256;
@@ -2277,6 +2281,8 @@ class Heap {
 
   MemoryChunk* chunks_queued_for_free_;
 
+  base::Semaphore pending_unmap_job_semaphore_;
+
   base::Mutex relocation_mutex_;
 
   int gc_callbacks_depth_;