[simulator] Make reference redirection thread-safe.

src/arm/simulator-arm.h

Index: src/arm/simulator-arm.h
diff --git a/src/arm/simulator-arm.h b/src/arm/simulator-arm.h
index 9c92c96830e4ae9a22e9011ff75127d9035a647c..35fc2f930e0ade4055b753837becc6eed6f7d945 100644
--- a/src/arm/simulator-arm.h
+++ b/src/arm/simulator-arm.h
@@ -361,7 +361,7 @@ class Simulator {
   static CachePage* GetCachePage(base::CustomMatcherHashMap* i_cache,
                                  void* page);
 
-  // Runtime call support.
+  // Runtime call support. Uses the isolate in a thread-safe way.
   static void* RedirectExternalReference(
       Isolate* isolate, void* external_function,
       v8::internal::ExternalReference::Type type);