[simulator] Make reference redirection thread-safe.

src/ppc/simulator-ppc.cc

Index: src/ppc/simulator-ppc.cc
diff --git a/src/ppc/simulator-ppc.cc b/src/ppc/simulator-ppc.cc
index fab4913bda2f67bd2fbc0938e82f17d951b3678d..853b2076ba9e3247689d375bc88f2ead10c14681 100644
--- a/src/ppc/simulator-ppc.cc
+++ b/src/ppc/simulator-ppc.cc
@@ -894,6 +894,8 @@ 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();
 }