Visit encountered JSWeakCollection list during scavenging.

src/heap.cc

Index: src/heap.cc
diff --git a/src/heap.cc b/src/heap.cc
index ff7a9ff12f20d6f7e37751f7d2bd91fb26939684..a530c396bedda23beec01b349f57c62f3760c944 100644
--- a/src/heap.cc
+++ b/src/heap.cc
@@ -145,6 +145,7 @@ Heap::Heap()
   set_native_contexts_list(NULL);
   set_array_buffers_list(Smi::FromInt(0));
   set_allocation_sites_list(Smi::FromInt(0));
+  set_encountered_weak_collections(Smi::FromInt(0));
   // Put a dummy entry in the remembered pages so we can find the list the
   // minidump even if there are no real unmapped pages.
   RememberUnmappedPage(NULL, false);
@@ -1508,6 +1509,9 @@ void Heap::Scavenge() {
     }
   }
 
+  // Copy objects reachable from the encountered weak collections list.
+  scavenge_visitor.VisitPointer(&encountered_weak_collections_);
+
   // Copy objects reachable from the code flushing candidates list.
   MarkCompactCollector* collector = mark_compact_collector();
   if (collector->is_code_flushing_enabled()) {