[simulator] Make reference redirection thread-safe.

src/mips/simulator-mips.h

Index: src/mips/simulator-mips.h
diff --git a/src/mips/simulator-mips.h b/src/mips/simulator-mips.h
index d32f6bc097d6c5b8e3731bbea97b23302d7407c2..1ed96bd00381ba98d968d7b891bf16b5961e63a8 100644
--- a/src/mips/simulator-mips.h
+++ b/src/mips/simulator-mips.h
@@ -463,7 +463,7 @@ class Simulator {
   // Exceptions.
   void SignalException(Exception e);
 
-  // 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);