[MD settings] icon color and sizing centralized

chrome/browser/resources/settings/controls/settings_dropdown_menu.html

 <link rel="import" href="chrome://resources/html/i18n_behavior.html">
 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-dropdown-menu/paper-dropdown-menu.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-item/paper-item.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-menu/paper-menu.html">
 <link rel="import" href="chrome://md-settings/controls/pref_control_behavior.html">
 <link rel="import" href="chrome://md-settings/prefs/pref_util.html">
 <link rel="import" href="chrome://md-settings/settings_shared_css.html">
 
 <dom-module id="settings-dropdown-menu">
   <template>
     <style include="settings-shared">
       paper-item {
         color: var(--paper-grey-800);
         font-size: inherit;
       }
 
       paper-dropdown-menu {
-        --iron-icon-fill-color: {
-          var(--paper-grey-800);
-        };
-        --iron-icon-stroke-color: {
-          var(--paper-grey-800);
-        };
+        --iron-icon-fill-color: var(--paper-grey-800);
+        --iron-icon-stroke-color: var(--paper-grey-800);
         --paper-font-subhead: {
           font-size: inherit;
         };
         --paper-input-container-underline: {
           background: var(--paper-grey-300);
         };
         width: 160px;
       }
      </style>
     <paper-dropdown-menu id="dropdownMenu" label="[[menuLabel_]]"
         on-iron-select="onSelect_" no-label-float$="[[noLabelFloat]]"
         disabled="[[shouldDisableMenu_(disabled, menuOptions.*)]]">
       <paper-menu class="dropdown-content" selected="{{selected_}}"
           attr-for-selected="data-value">
         <template is="dom-repeat" items="[[menuOptions]]">
           <paper-item data-value$="[[item.value]]">[[item.name]]</paper-item>
         </template>
         <paper-item data-value$="[[notFoundValue_]]"
             hidden$="[[!isSelectedNotFound_(selected_)]]">
           [[i18n('custom')]]
         </paper-item>
       </paper-menu>
     </paper-dropdown-menu>
   </template>
   <script src="settings_dropdown_menu.js"></script>
 </dom-module>