Fix corner-case in heap size estimation.

src/spaces.cc

Index: src/spaces.cc
diff --git a/src/spaces.cc b/src/spaces.cc
index c96388b238293c148a7e24c72d760a4b539d5cb2..82852c71b0841e5b611de35bcd1384078a1a4870 100644
--- a/src/spaces.cc
+++ b/src/spaces.cc
@@ -881,6 +881,8 @@ void PagedSpace::ReleasePage(Page* page) {
     intptr_t size = free_list_.EvictFreeListItems(page);
     accounting_stats_.AllocateBytes(size);
     ASSERT_EQ(Page::kObjectAreaSize, static_cast<int>(size));
+  } else {
+    DecreaseUnsweptFreeBytes(page);
   }
 
   if (Page::FromAllocationTop(allocation_info_.top) == page) {
@@ -2280,7 +2282,7 @@ bool PagedSpace::AdvanceSweeper(intptr_t bytes_to_sweep) {
         PrintF("Sweeping 0x%" V8PRIxPTR " lazily advanced.\n",
                reinterpret_cast<intptr_t>(p));
       }
-      unswept_free_bytes_ -= (Page::kObjectAreaSize - p->LiveBytes());
+      DecreaseUnsweptFreeBytes(p);
       freed_bytes += MarkCompactCollector::SweepConservatively(this, p);
     }
     p = next_page;