When adjusting page's live bytes counter from the mutator adjust owners unswept free bytes counter.

src/spaces.h

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 454 matching lines @@
              ((by < 0) ? '-' : '+'), ((by < 0) ? -by : by),
              live_byte_count_ + by);
     }
     live_byte_count_ += by;
     ASSERT_LE(static_cast<unsigned>(live_byte_count_), size_);
   }
   int LiveBytes() {
     ASSERT(static_cast<unsigned>(live_byte_count_) <= size_);
     return live_byte_count_;
   }
-  static void IncrementLiveBytes(Address address, int by) {
+
+  static void IncrementLiveBytesFromGC(Address address, int by) {
     MemoryChunk::FromAddress(address)->IncrementLiveBytes(by);
   }
 
+  static void IncrementLiveBytesFromMutator(Address address, int by);
+
   static const intptr_t kAlignment =
       (static_cast<uintptr_t>(1) << kPageSizeBits);
 
   static const intptr_t kAlignmentMask = kAlignment - 1;
 
   static const intptr_t kSizeOffset = kPointerSize + kPointerSize;
 
   static const intptr_t kLiveBytesOffset =
       kSizeOffset + kPointerSize + kPointerSize + kPointerSize +
       kPointerSize + kPointerSize + kPointerSize + kIntSize;
@@ skipping 1067 matching lines @@
            !p->IsFlagSet(Page::RESCAN_ON_EVACUATION) &&
            !p->WasSweptPrecisely();
   }
 
   void SetPagesToSweep(Page* first) {
     ASSERT(unswept_free_bytes_ == 0);
     if (first == &anchor_) first = NULL;
     first_unswept_page_ = first;
   }
 
+  void IncrementUnsweptFreeBytes(int by) {
+    unswept_free_bytes_ += by;
+  }
+
   void IncreaseUnsweptFreeBytes(Page* p) {
     ASSERT(ShouldBeSweptLazily(p));
     unswept_free_bytes_ += (Page::kObjectAreaSize - p->LiveBytes());
   }
 
   void DecreaseUnsweptFreeBytes(Page* p) {
     ASSERT(ShouldBeSweptLazily(p));
     unswept_free_bytes_ -= (Page::kObjectAreaSize - p->LiveBytes());
   }
 
@@ skipping 1073 matching lines @@
   }
   // Must be small, since an iteration is used for lookup.
   static const int kMaxComments = 64;
 };
 #endif
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_SPACES_H_