[heap] Verify mark bits when iterating mark bits.

src/heap/mark-compact.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_MARK_COMPACT_H_
 #define V8_HEAP_MARK_COMPACT_H_
 
 #include "src/base/bits.h"
 #include "src/heap/spaces.h"
 
@@ skipping 811 matching lines @@
     DCHECK(cell_index_ == Bitmap::IndexToCell(Bitmap::CellAlignIndex(
                               chunk_->AddressToMarkbitIndex(cell_base_))));
     return cell_base_;
   }
 
   inline void Advance() {
     cell_index_++;
     cell_base_ += 32 * kPointerSize;
   }
 
+  // Return the next mark bit cell. If there is no next it returns 0;
+  inline MarkBit::CellType PeekNext() {
+    if (HasNext()) {
+      return cells_[cell_index_ + 1];
+    }
+    return 0;
+  }
+
  private:
   MemoryChunk* chunk_;
   MarkBit::CellType* cells_;
   unsigned int last_cell_index_;
   unsigned int cell_index_;
   Address cell_base_;
 };
 
 
 class EvacuationScope BASE_EMBEDDED {
  public:
   explicit EvacuationScope(MarkCompactCollector* collector)
       : collector_(collector) {
     collector_->set_evacuation(true);
   }
 
   ~EvacuationScope() { collector_->set_evacuation(false); }
 
  private:
   MarkCompactCollector* collector_;
 };
 
 
 const char* AllocationSpaceName(AllocationSpace space);
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_HEAP_MARK_COMPACT_H_