MD Settings: add a <controlled-button> that uses prefs to disable itself

chrome/browser/resources/settings/downloads_page/downloads_page.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/paper-button/paper-button.html">
+<link rel="import" href="/controls/controlled_button.html">
 <link rel="import" href="/controls/settings_checkbox.html">
 <link rel="import" href="/controls/settings_input.html">
 <link rel="import" href="/settings_shared_css.html">
 
 <dom-module id="settings-downloads-page">
   <template>
     <style include="settings-shared">
       settings-input {
         /* This ensures that the "Download location" label and the text in the
          * input itself are vertically aligned. */
         margin-bottom: 1em;
         width: 300px;
       }
 
       #locationLabel {
         -webkit-margin-end: 10px;
       }
     </style>
     <div class="settings-box first two-line">
       <div class="start">
         <div>$i18n{downloadLocation}</div>
         <div class="secondary">[[prefs.download.default_directory.value]]</div>
       </div>
       <div class="secondary-action">
-        <paper-button class="secondary-button" id="changeDownloadsPath"
-            on-tap="selectDownloadLocation_">
+        <controlled-button class="secondary-button" id="changeDownloadsPath"
+            on-tap="selectDownloadLocation_"
+            pref="[[prefs.download.default_directory]]">
           $i18n{changeDownloadLocation}
-        </paper-button>
+        </controlled-button>
       </div>
     </div>
     <div class="settings-box block">
       <settings-checkbox pref="{{prefs.download.prompt_for_download}}"
           label="$i18n{promptForDownload}">
       </settings-checkbox>
 <if expr="chromeos">
       <settings-checkbox pref="{{prefs.gdata.disabled}}"
           label="$i18n{disconnectGoogleDriveAccount}">
       </settings-checkbox>
 </if>
     </div>
   </template>
   <script src="downloads_page.js"></script>
 </dom-module>