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 f974c0764b09dc9190e680d429a4c58950e57f22..48b0c6fe5bcfeaceee9c5521d132f070b53b15ce 100644
--- a/src/heap/heap.h
+++ b/src/heap/heap.h
@@ -1125,7 +1125,9 @@ class Heap {
 
   void QueueMemoryChunkForFree(MemoryChunk* chunk);
   void FilterStoreBufferEntriesOnAboutToBeFreedPages();
+  void FreeQueuedChunks(MemoryChunk* list_head);
   void FreeQueuedChunks();
+  void WaitUntilUnmappingOfFreeChunksCompleted();
 
   bool RecentIdleNotificationHappened();
 
@@ -1656,6 +1658,8 @@ class Heap {
 #endif
 
  private:
+  class UnmapFreeMemoryTask;
+
   struct StrongRootsList;
 
   struct StringTypeTable {
@@ -2337,6 +2341,8 @@ class Heap {
 
   MemoryChunk* chunks_queued_for_free_;
 
+  base::Semaphore pending_unmap_job_semaphore_;
+
   base::Mutex relocation_mutex_;
 
   int gc_callbacks_depth_;