Do not report unessential weak references in heap snapshot.

src/heap-snapshot-generator.cc

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 1663 matching lines @@
 }
 
 
 void V8HeapExplorer::SetWeakReference(HeapObject* parent_obj,
                                       int parent_entry,
                                       int index,
                                       Object* child_obj,
                                       int field_offset) {
   ASSERT(parent_entry == GetEntry(parent_obj)->index());
   HeapEntry* child_entry = GetEntry(child_obj);
-  if (child_entry != NULL) {
+  if (child_entry == NULL) return;
+  if (IsEssentialObject(child_obj)) {
     filler_->SetIndexedReference(HeapGraphEdge::kWeak,
                                  parent_entry,
                                  index,
                                  child_entry);
-    IndexedReferencesExtractor::MarkVisitedField(parent_obj, field_offset);
   }
+  IndexedReferencesExtractor::MarkVisitedField(parent_obj, field_offset);
 }
 
 
 void V8HeapExplorer::SetPropertyReference(HeapObject* parent_obj,
                                           int parent_entry,
                                           Name* reference_name,
                                           Object* child_obj,
                                           const char* name_format_string,
                                           int field_offset) {
   ASSERT(parent_entry == GetEntry(parent_obj)->index());
@@ skipping 987 matching lines @@
 
 
 void HeapSnapshotJSONSerializer::SortHashMap(
     HashMap* map, List<HashMap::Entry*>* sorted_entries) {
   for (HashMap::Entry* p = map->Start(); p != NULL; p = map->Next(p))
     sorted_entries->Add(p);
   sorted_entries->Sort(SortUsingEntryValue);
 }
 
 } }  // namespace v8::internal