[simulator] Make reference redirection thread-safe.

src/isolate.h

Index: src/isolate.h
diff --git a/src/isolate.h b/src/isolate.h
index 033ca1019168d8c1c1aab49253497b52d4a89955..f3ea9a6fcc191b6a25fbd3c5d2a6d62a50cfc458 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -1250,6 +1250,9 @@ class Isolate {
 
 #ifdef USE_SIMULATOR
   base::Mutex* simulator_i_cache_mutex() { return &simulator_i_cache_mutex_; }
+  base::Mutex* simulator_redirection_mutex() {
+    return &simulator_redirection_mutex_;
+  }
 #endif
 
   void set_allow_atomics_wait(bool set) { allow_atomics_wait_ = set; }
@@ -1570,6 +1573,7 @@ class Isolate {
 
 #ifdef USE_SIMULATOR
   base::Mutex simulator_i_cache_mutex_;
+  base::Mutex simulator_redirection_mutex_;
 #endif
 
   bool allow_atomics_wait_;