[heap] Implement simple concurrent marking deque.

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 324 matching lines @@
   V(UninitializedMap)                   \
   V(UninitializedValue)                 \
   V(WeakCellMap)                        \
   V(WithContextMap)                     \
   PRIVATE_SYMBOL_LIST(V)
 
 // Forward declarations.
 class AllocationObserver;
 class ArrayBufferTracker;
 class ConcurrentMarking;
+class ConcurrentMarkingDeque;
 class GCIdleTimeAction;
 class GCIdleTimeHandler;
 class GCIdleTimeHeapState;
 class GCTracer;
 class HeapObjectsFilter;
 class HeapStats;
 class HistogramTimer;
 class Isolate;
 class LocalEmbedderHeapTracer;
 class MemoryAllocator;
@@ skipping 2304 matching lines @@
   friend class PagedSpace;
   DISALLOW_COPY_AND_ASSIGN(AllocationObserver);
 };
 
 V8_EXPORT_PRIVATE const char* AllocationSpaceName(AllocationSpace space);
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_HEAP_HEAP_H_