Wait for concurrent unmapping tasks in GC prologue.

src/heap/heap.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_HEAP_HEAP_H_
 #define V8_HEAP_HEAP_H_
 
 #include <cmath>
 #include <map>
 
@@ skipping 2358 matching lines @@
   // Currently set GC callback flags that are used to pass information between
   // the embedder and V8's GC.
   GCCallbackFlags current_gc_callback_flags_;
 
   ExternalStringTable external_string_table_;
 
   VisitorDispatchTable<ScavengingCallback> scavenging_visitors_table_;
 
   MemoryChunk* chunks_queued_for_free_;
 
-  base::Semaphore pending_unmap_job_semaphore_;
+  size_t concurrent_unmapping_tasks_active_;
+
+  base::Semaphore pending_unmapping_tasks_semaphore_;
 
   base::Mutex relocation_mutex_;
 
   int gc_callbacks_depth_;
 
   bool deserialization_complete_;
 
   bool concurrent_sweeping_enabled_;
 
   // |live_array_buffers_| maps externally allocated memory used as backing
@@ skipping 396 matching lines @@
   DisallowHeapAllocation no_allocation;  // i.e. no gc allowed.
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(PathTracer);
 };
 #endif  // DEBUG
 }
 }  // namespace v8::internal
 
 #endif  // V8_HEAP_HEAP_H_