[simulator] Make reference redirection thread-safe.

src/mips64/simulator-mips64.cc

Index: src/mips64/simulator-mips64.cc
diff --git a/src/mips64/simulator-mips64.cc b/src/mips64/simulator-mips64.cc
index 5641c10818a9d12d7bfea5951d6537a9fb70d20e..c0a1e6e868f1624ffd799dff81e2d8ca4384b130 100644
--- a/src/mips64/simulator-mips64.cc
+++ b/src/mips64/simulator-mips64.cc
@@ -949,6 +949,8 @@ 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();
 }