fix mixin host selector

Author: notwaldorf

element-styles/paper-item.html

@@ -35,7 +35,7 @@
 <dom-module id="paper-item">
   <template>
     <style>
-      html {
+      html, :host {
         --paper-item: {
           display: block;
           position: relative;

element-styles/paper-material.html

@@ -34,7 +34,7 @@
 <dom-module id="paper-material">
   <template>
     <style>
-      html {
+      html, :host {
         --paper-material: {
           display: block;
           position: relative;