Fix corner-case in heap size estimation.

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 1569 matching lines @@
            !p->WasSweptPrecisely();
   }
 
   void SetPagesToSweep(Page* first) {
     ASSERT(unswept_free_bytes_ == 0);
     if (first == &anchor_) first = NULL;
     first_unswept_page_ = first;
   }
 
   void MarkPageForLazySweeping(Page* p) {
+    ASSERT(ShouldBeSweptLazily(p));
     unswept_free_bytes_ += (Page::kObjectAreaSize - p->LiveBytes());
   }
 
+  void UnmarkPageForLazySweeping(Page* p) {
+    ASSERT(ShouldBeSweptLazily(p));
+    unswept_free_bytes_ -= (Page::kObjectAreaSize - p->LiveBytes());
+  }
+
   bool AdvanceSweeper(intptr_t bytes_to_sweep);
 
   bool IsSweepingComplete() {
     return !first_unswept_page_->is_valid();
   }
 
   inline bool HasAPage() { return anchor_.next_page() != &anchor_; }
   Page* FirstPage() { return anchor_.next_page(); }
   Page* LastPage() { return anchor_.prev_page(); }
 
@@ skipping 1048 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_