[heap] Verify mark bits when iterating mark bits.

src/heap/mark-compact.h

Index: src/heap/mark-compact.h
diff --git a/src/heap/mark-compact.h b/src/heap/mark-compact.h
index 95f2e09d066a2d653c37e43d1f7764944d241a87..322e8b83869295a338b6ba6e6b0b44d6291a1650 100644
--- a/src/heap/mark-compact.h
+++ b/src/heap/mark-compact.h
@@ -829,6 +829,14 @@ class MarkBitCellIterator BASE_EMBEDDED {
     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_;