Fix assert in PagedSpace::SizeOfObject for concurrent sweeping.

src/spaces.cc

Index: src/spaces.cc
diff --git a/src/spaces.cc b/src/spaces.cc
index 91f8b81ed8c27be4702ee51b9a14d2dca1f6f8fe..0af587d2052263e6168b65cb1573ceb5a99203c4 100644
--- a/src/spaces.cc
+++ b/src/spaces.cc
@@ -2476,6 +2476,12 @@ bool PagedSpace::ReserveSpace(int size_in_bytes) {
 }
 
 
+intptr_t PagedSpace::SizeOfObjects() {
+  ASSERT(!heap()->IsSweepingComplete() || (unswept_free_bytes_ == 0));
+  return Size() - unswept_free_bytes_ - (limit() - top());
+}
+
+
 // After we have booted, we have created a map which represents free space
 // on the heap.  If there was already a free list then the elements on it
 // were created with the wrong FreeSpaceMap (normally NULL), so we need to