If we want to use application system to get application service , there is two options: 
1. Provide an instance of ApplicationSystem or RuntimeContext in XwalkExtensionProcessHost constructor. This need modify constructor of XwalkExtensionProcessHost. Also we need modify constructor of  XWalkExtensionService. Becasue  XwalkExtensionProcessHost instance is created from  XWalkExtensionService, which constructor does't contain ApplicationSystem or RuntimeContext either.  
 2. Add a new static method "Get()" in application system.  But what's the difference than adding static function to application service?