Fix corner-case in heap size estimation.

src/spaces.h

Index: src/spaces.h
diff --git a/src/spaces.h b/src/spaces.h
index f067398f50db54723293f4f05286ecda98c8783d..92bd91208b0cbb5af67f1c8e5391b78562e0a022 100644
--- a/src/spaces.h
+++ b/src/spaces.h
@@ -1587,9 +1587,15 @@ class PagedSpace : public Space {
   }
 
   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() {