Prevent _showHideChildren from being called on placeholders.
Prevent _showHideChildren from being called on placeholders.

This CL fixes issue #4096 where certain cases could cause
_showHideChildren to get called on template placeholders.

gronke/polymer,Polymer/polymer,yforrest/polymer,JeremybellEU/polymer,gronke/polymer,yforrest/polymer,JeremybellEU/polymer,gronke/polymer,yforrest/polymer,ikeagold/polymer,Polymer/polymer,ikeagold/polymer,JeremybellEU/polymer,Polymer/polymer,ikeagold/polymer