Make heap size estimation more accurate.

src/spaces.h

Index: src/spaces.h
diff --git a/src/spaces.h b/src/spaces.h
index 41bfec90f142797bdc295d622505ee963c6b05c6..60432e39cf7c30a55da8131e52fba42350705b7e 100644
--- a/src/spaces.h
+++ b/src/spaces.h
@@ -1469,9 +1469,16 @@ class PagedSpace : public Space {
   // linear allocation area (between top and limit) are also counted here.
   virtual intptr_t Size() { return accounting_stats_.Size(); }
 
-  // As size, but the bytes in the current linear allocation area are not
-  // included.
-  virtual intptr_t SizeOfObjects() { return Size() - (limit() - top()); }
+  // As size, but the bytes in lazily swept pages are estimated and the bytes
+  // in the current linear allocation area are not included.
+  intptr_t SizeOfObjectsSlow();
+  virtual intptr_t SizeOfObjects() {
+    if (IsSweepingComplete()) {
+      return Size() - (limit() - top());
+    } else {
+      return SizeOfObjectsSlow();
+    }
+  }
 
   // Wasted bytes in this space.  These are just the bytes that were thrown away
   // due to being too small to use for allocation.  They do not include the
@@ -1479,9 +1486,7 @@ class PagedSpace : public Space {
   virtual intptr_t Waste() { return accounting_stats_.Waste(); }
 
   // Returns the allocation pointer in this space.
-  Address top() {
-    return allocation_info_.top;
-  }
+  Address top() { return allocation_info_.top; }
   Address limit() { return allocation_info_.limit; }
 
   // Allocate the requested number of bytes in the space if possible, return a