Properly shut down SpiderMonkey using JS_ShutDown.

This also adds some validation to ensure the correct order of JS_Init,
JS_NewRuntime, JS_DestroyRuntime and JS_ShutDown calls.

Refs #3708

This was SVN commit r18584.

source/scriptinterface/ScriptRuntime.h

@@ -79,7 +79,6 @@ private:
 	
 	std::list<JSContext*> m_Contexts;
 	std::vector<std::shared_ptr<void> > m_FinalizationListObjectIdCache;
-	static bool m_Initialized;
 
 	int m_RuntimeSize;
 	int m_HeapGrowthBytesGCTrigger;