[MD settings] show icon when slider controlled by something

chrome/browser/resources/settings/appearance_page/appearance_fonts_page.html

 <link rel="import" href="chrome://resources/cr_elements/cr_slider/cr_slider.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/html/web_ui_listener_behavior.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-icon-button/paper-icon-button-light.html">
 <link rel="import" href="fonts_browser_proxy.html">
 <link rel="import" href="../controls/settings_dropdown_menu.html">
 <link rel="import" href="../i18n_setup.html">
 <link rel="import" href="../settings_shared_css.html">
 
 <dom-module id="settings-appearance-fonts-page">
   <template>
     <style include="settings-shared"></style>
     <div class="settings-box first">
       <div class="start">$i18n{fontSize}</div>
       <cr-slider id="sizeSlider"
-          value="{{prefs.webkit.webprefs.default_font_size.value}}"
+          pref="{{prefs.webkit.webprefs.default_font_size}}"
           tick-values="[[fontSizeRange_]]"
           label-min="$i18n{tiny}" label-max="$i18n{huge}">
       </cr-slider>
     </div>
     <div class="settings-box">
       <div class="start">$i18n{minimumFont}</div>
       <div class="list-item"
           style$="[[computeStyle_(
               prefs.webkit.webprefs.minimum_font_size.value,
               prefs.webkit.webprefs.fonts.standard.Zyyy.value)]]">
         <span>[[prefs.webkit.webprefs.minimum_font_size.value]]</span>
         : $i18n{quickBrownFox}
       </div>
       <cr-slider id="minimumSizeSlider"
-          value="{{prefs.webkit.webprefs.minimum_font_size.value}}"
+          pref="{{prefs.webkit.webprefs.minimum_font_size}}"
           tick-values="[[minimumFontSizeRange_]]"
           label-min="$i18n{tiny}" label-max="$i18n{huge}">
       </cr-slider>
     </div>
     <div class="settings-box">
       <div class="start">
         <h2>$i18n{standardFont}</h2>
         <div class="list-frame">
           <div class="list-item">
             <settings-dropdown-menu class="start"
@@ skipping 77 matching lines @@
           </div>
         </div>
         <button class="icon-external" is="paper-icon-button-light"
             aria-label="$i18n{advancedFontSettings}"
             aria-describedby="advancedButtonSublabel"></button>
       </div>
     </template>
   </template>
   <script src="appearance_fonts_page.js"></script>
 </dom-module>