Use hash map to look for objects in the root array when serializing.

src/heap/heap.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/v8.h"
 
 #include "src/accessors.h"
 #include "src/api.h"
 #include "src/base/bits.h"
 #include "src/base/once.h"
@@ skipping 2887 matching lines @@
   set_regexp_multiple_cache(*factory->NewFixedArray(
       RegExpResultsCache::kRegExpResultsCacheSize, TENURED));
 
   // Allocate cache for external strings pointing to native source code.
   set_natives_source_cache(
       *factory->NewFixedArray(Natives::GetBuiltinsCount()));
 
   set_undefined_cell(*factory->NewCell(factory->undefined_value()));
 
   // The symbol registry is initialized lazily.
-  set_symbol_registry(undefined_value());
+  set_symbol_registry(Smi::FromInt(0));
 
   // Allocate object to hold object observation state.
   set_observation_state(*factory->NewJSObjectFromMap(
       factory->NewMap(JS_OBJECT_TYPE, JSObject::kHeaderSize)));
 
   // Microtask queue uses the empty fixed array as a sentinel for "empty".
   // Number of queued microtasks stored in Isolate::pending_microtask_count().
   set_microtask_queue(empty_fixed_array());
 
   Handle<SeededNumberDictionary> slow_element_dictionary =
@@ skipping 3285 matching lines @@
       static_cast<int>(object_sizes_last_time_[index]));
   CODE_AGE_LIST_COMPLETE(ADJUST_LAST_TIME_OBJECT_COUNT)
 #undef ADJUST_LAST_TIME_OBJECT_COUNT
 
   MemCopy(object_counts_last_time_, object_counts_, sizeof(object_counts_));
   MemCopy(object_sizes_last_time_, object_sizes_, sizeof(object_sizes_));
   ClearObjectStats();
 }
 }
 }  // namespace v8::internal