Fix a race condition in simulator that happens when flushing icache.

src/isolate.h

Index: src/isolate.h
diff --git a/src/isolate.h b/src/isolate.h
index b9ba54ea463462bbd8635afa6a855789a5bdadf0..10c151efe7d048638ba214d7ee2ea97d071da953 100644
--- a/src/isolate.h
+++ b/src/isolate.h
@@ -1164,6 +1164,10 @@ class Isolate {
 
   PRINTF_FORMAT(2, 3) void PrintWithTimestamp(const char* format, ...);
 
+#ifdef USE_SIMULATOR
+  base::Mutex* simulator_i_cache_mutex() { return &simulator_i_cache_mutex_; }
+#endif
+
  protected:
   explicit Isolate(bool enable_serializer);
   bool IsArrayOrObjectPrototype(Object* object);
@@ -1440,6 +1444,10 @@ class Isolate {
   v8::Isolate::AbortOnUncaughtExceptionCallback
       abort_on_uncaught_exception_callback_;
 
+#ifdef USE_SIMULATOR
+  base::Mutex simulator_i_cache_mutex_;
+#endif
+
   friend class ExecutionAccess;
   friend class HandleScopeImplementer;
   friend class OptimizingCompileDispatcher;