[simulator] Make reference redirection thread-safe.

src/mips/simulator-mips.cc

Index: src/mips/simulator-mips.cc
diff --git a/src/mips/simulator-mips.cc b/src/mips/simulator-mips.cc
index cf78f8001c9dcc6d86e63fa17aec4ab6ed7912e9..38816e9e0d60d8093f69be974378a638f12579a1 100644
--- a/src/mips/simulator-mips.cc
+++ b/src/mips/simulator-mips.cc
@@ -1016,6 +1016,8 @@ void Simulator::TearDown(base::CustomMatcherHashMap* i_cache,
 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_swi_instruction();
 }