MD Settings: Associate suppages with the controls that trigger them.

chrome/browser/resources/settings/appearance_page/appearance_page.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/iron-flex-layout/classes/iron-flex-layout.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-icon/iron-icon.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/neon-animation/neon-animatable.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-button/paper-button.html">
 <link rel="import" href="/appearance_page/appearance_browser_proxy.html">
 <link rel="import" href="/controls/controlled_radio_button.html">
 <link rel="import" href="/controls/settings_dropdown_menu.html">
 <link rel="import" href="/controls/settings_input.html">
@@ skipping 93 matching lines @@
           </settings-checkbox>
         </div>
         <div class="settings-box">
 </if>
           <div class="start">$i18n{fontSize}</div>
           <settings-dropdown-menu id="defaultFontSize"
               pref="{{prefs.webkit.webprefs.default_font_size}}"
               menu-options="[[fontSizeOptions_]]" no-label-float>
           </settings-dropdown-menu>
         </div>
-        <div class="settings-box two-line single-column"
+        <div id="customize-fonts-subpage-trigger"
+            class="settings-box two-line single-column"
             on-tap="onCustomizeFontsTap_" actionable>
           <div>$i18n{customizeFonts}</div>
           <div class="secondary">$i18n{chooseFontsAndEncoding}</div>
         </div>
         <div class="settings-box"
             hidden="[[!pageVisibility.pageZoom]]">
           <div class="start">$i18n{pageZoom}</div>
           <settings-dropdown-menu id="pageZoom" pref="{{defaultZoomLevel_}}"
               menu-options="[[pageZoomOptions_]]" no-label-float>
           </settings-dropdown-menu>
         </div>
       </neon-animatable>
       <template is="dom-if" name="appearance-fonts">
-        <settings-subpage page-title="$i18n{customizeFonts}">
+        <settings-subpage
+            associated-control="[[$$('#customize-fonts-subpage-trigger')]]"
+            page-title="$i18n{customizeFonts}">
           <settings-appearance-fonts-page prefs="{{prefs}}">
           </settings-appearance-fonts-page>
         </settings-subpage>
       </template>
     </settings-animated-pages>
   </template>
   <script src="appearance_page.js"></script>
 </dom-module>