Don`t use references to JsEngine to avoid use-after-free errors,switch to shared_ptr instead

shell/src/GcCommand.h

 #ifndef GC_COMMAND_H
 #define GC_COMMAND_H
 
 #include <AdblockPlus.h>
 #include <string>
 
 #include "Command.h"
 
 class GcCommand : public Command
 {
 public:
-  explicit GcCommand(AdblockPlus::JsEngine& jsEngine);
+  explicit GcCommand(AdblockPlus::JsEnginePtr jsEngine);
   void operator()(const std::string& arguments);
   std::string GetDescription() const;
   std::string GetUsage() const;
 
 private:
-  AdblockPlus::JsEngine& jsEngine;
+  AdblockPlus::JsEnginePtr jsEngine;
 };
 
 #endif