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..e89c6801c25b58ebd3bb3517d2771ae2427f3443 100644
--- a/src/spaces.h
+++ b/src/spaces.h
@@ -1586,10 +1586,16 @@ class PagedSpace : public Space {
     first_unswept_page_ = first;
   }
 
-  void MarkPageForLazySweeping(Page* p) {
+  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());
+  }
+
   bool AdvanceSweeper(intptr_t bytes_to_sweep);
 
   bool IsSweepingComplete() {