[simulator] Make reference redirection thread-safe.

src/arm64/simulator-arm64.cc

Index: src/arm64/simulator-arm64.cc
diff --git a/src/arm64/simulator-arm64.cc b/src/arm64/simulator-arm64.cc
index 59fa0fdbfc569c9ff49ce60d7dc5c97c709970f8..fb0e614982919bb0ef60608597f0f7375d28ba1f 100644
--- a/src/arm64/simulator-arm64.cc
+++ b/src/arm64/simulator-arm64.cc
@@ -801,6 +801,8 @@ void Simulator::DoRuntimeCall(Instruction* instr) {
 void* Simulator::RedirectExternalReference(Isolate* isolate,
                                            void* external_function,
                                            ExternalReference::Type type) {
+  base::LockGuard<base::Mutex> lock_guard(
+      isolate->simulator_redirection_mutex());
   Redirection* redirection = Redirection::Get(isolate, external_function, type);
   return redirection->address_of_redirect_call();
 }