d8 workers: make sure Shell::Quit is only called once

src/d8.h

Index: src/d8.h
diff --git a/src/d8.h b/src/d8.h
index 6c632c9709ca2980e2ec4e9b9240e15e18a8ee00..f126dd220a25731a5071387d2259bd6a0bd8a572 100644
--- a/src/d8.h
+++ b/src/d8.h
@@ -15,6 +15,9 @@
 #include "src/base/compiler-specific.h"
 #endif  // !V8_SHARED
 
+#include "src/base/once.h"
+
+
 namespace v8 {
 
 
@@ -411,6 +414,7 @@ class Shell : public i::AllStatic {
 
   static void Print(const v8::FunctionCallbackInfo<v8::Value>& args);
   static void Write(const v8::FunctionCallbackInfo<v8::Value>& args);
+  static void QuitOnce(v8::FunctionCallbackInfo<v8::Value>* args);
   static void Quit(const v8::FunctionCallbackInfo<v8::Value>& args);
   static void Version(const v8::FunctionCallbackInfo<v8::Value>& args);
   static void Read(const v8::FunctionCallbackInfo<v8::Value>& args);
@@ -468,6 +472,7 @@ class Shell : public i::AllStatic {
 
  private:
   static Persistent<Context> evaluation_context_;
+  static base::OnceType quit_once_;
 #ifndef V8_SHARED
   static Persistent<Context> utility_context_;
   static CounterMap* counter_map_;