[simulator] Make reference redirection thread-safe.

src/arm64/simulator-arm64.h

 // Copyright 2013 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.
 
 #ifndef V8_ARM64_SIMULATOR_ARM64_H_
 #define V8_ARM64_SIMULATOR_ARM64_H_
 
 #include <stdarg.h>
 #include <vector>
 
@@ skipping 255 matching lines @@
   uintptr_t PushAddress(uintptr_t address);
 
   // Pop an address from the JS stack.
   uintptr_t PopAddress();
 
   // Accessor to the internal simulator stack area.
   uintptr_t StackLimit(uintptr_t c_limit) const;
 
   void ResetState();
 
-  // Runtime call support.
+  // Runtime call support. Uses the isolate in a thread-safe way.
   static void* RedirectExternalReference(Isolate* isolate,
                                          void* external_function,
                                          ExternalReference::Type type);
   void DoRuntimeCall(Instruction* instr);
 
   // Run the simulator.
   static const Instruction* kEndOfSimAddress;
   void DecodeInstruction();
   void Run();
   void RunFrom(Instruction* start);
@@ skipping 705 matching lines @@
     Simulator::current(isolate)->PopAddress();
   }
 };
 
 #endif  // !defined(USE_SIMULATOR)
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_ARM64_SIMULATOR_ARM64_H_