Do not force GC on each objects tracking sample.

src/heap-snapshot-generator.h

 // 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 261 matching lines @@
     return ComputeIntegerHash(
         static_cast<uint32_t>(reinterpret_cast<uintptr_t>(addr)),
         v8::internal::kZeroHashSeed);
   }
 
   SnapshotObjectId next_id_;
   HashMap entries_map_;
   List<EntryInfo> entries_;
   List<TimeInterval> time_intervals_;
   Heap* heap_;
+  double last_gc_time_;
 
   DISALLOW_COPY_AND_ASSIGN(HeapObjectsMap);
 };
 
 
 class HeapSnapshotsCollection {
  public:
   explicit HeapSnapshotsCollection(Heap* heap);
   ~HeapSnapshotsCollection();
 
@@ skipping 388 matching lines @@
   friend class HeapSnapshotJSONSerializerIterator;
 
   DISALLOW_COPY_AND_ASSIGN(HeapSnapshotJSONSerializer);
 };
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_HEAP_SNAPSHOT_GENERATOR_H_