Do not visit smis in the root list during GC.

src/serialize.h

 // Copyright 2012 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 561 matching lines @@
 
   // Some roots should not be serialized, because their actual value depends on
   // absolute addresses and they are reset after deserialization, anyway.
   bool ShouldBeSkipped(Object** current);
 
   Isolate* isolate_;
   // Keep track of the fullness of each space in order to generate
   // relative addresses for back references.
   int fullness_[LAST_SPACE + 1];
   SnapshotByteSink* sink_;
-  int current_root_index_;
   ExternalReferenceEncoder* external_reference_encoder_;
   static bool serialization_enabled_;
   // Did we already make use of the fact that serialization was not enabled?
   static bool too_late_to_enable_now_;
   SerializationAddressMapper address_mapper_;
   intptr_t root_index_wave_front_;
   void Pad();
 
   friend class ObjectSerializer;
   friend class Deserializer;
@@ skipping 71 matching lines @@
  private:
   virtual bool ShouldBeInThePartialSnapshotCache(HeapObject* o) {
     return false;
   }
 };
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_SERIALIZE_H_