[heap] Implement simple concurrent marking deque.

src/heap/sequential-marking-deque.h

 // Copyright 2017 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_SEQUENTIAL_MARKING_DEQUE_
 #define V8_HEAP_SEQUENTIAL_MARKING_DEQUE_
 
 #include <deque>
 
 #include "src/base/platform/mutex.h"
@@ skipping 73 matching lines @@
     if (IsFull()) {
       SetOverflowed();
       return false;
     } else {
       bottom_ = ((bottom_ - 1) & mask_);
       array_[bottom_] = object;
       return true;
     }
   }
 
-  template <typename Callback>
-  void Iterate(Callback callback) {
-    int i = bottom_;
-    while (i != top_) {
-      callback(array_[i]);
-      i = (i + 1) & mask_;
-    }
-  }
-
   // Calls the specified callback on each element of the deque and replaces
   // the element with the result of the callback. If the callback returns
   // nullptr then the element is removed from the deque.
   // The callback must accept HeapObject* and return HeapObject*.
   template <typename Callback>
   void Update(Callback callback) {
     int i = bottom_;
     int new_top = bottom_;
     while (i != top_) {
       HeapObject* object = callback(array_[i]);
@@ skipping 59 matching lines @@
   bool uncommit_task_pending_;
   Heap* heap_;
 
   DISALLOW_COPY_AND_ASSIGN(SequentialMarkingDeque);
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_SEQUENTIAL_MARKING_DEQUE_