MD Settings: Allow controlled-button to be used with icons.

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="../controls/controlled_button.html">
 <link rel="import" href="../controls/settings_toggle_button.html">
 <link rel="import" href="../settings_shared_css.html">
 
 <dom-module id="settings-downloads-page">
   <template>
-    <style include="settings-shared"></style>
-    <div class="settings-box first two-line">
+    <style include="settings-shared">
+      .settings-box[disabled] {
+        pointer-events: none;
+      }
+    </style>
+    <div class="settings-box first two-line" on-tap="selectDownloadLocation_"
+        disabled$="[[isControlled_]]" actionable>
       <div class="start">
         <div>$i18n{downloadLocation}</div>
         <div class="secondary">
 <if expr="not chromeos">
           [[prefs.download.default_directory.value]]
 </if>
 <if expr="chromeos">
           [[getDownloadLocation_(prefs.download.default_directory.value)]]
 </if>
         </div>
       </div>
-      <div class="secondary-action">
-        <controlled-button class="secondary-button" id="changeDownloadsPath"
-            on-tap="selectDownloadLocation_"
-            pref="[[prefs.download.default_directory]]"
-            end-justified>
-          $i18n{changeDownloadLocation}
-        </controlled-button>
-      </div>
+      <controlled-button id="changeDownloadsPath" icon-class="icon-external"
+          pref="[[prefs.download.default_directory]]" end-justified
+          controlled="{{isControlled_}}">
+      </controlled-button>
     </div>
     <div class="settings-box block">
       <settings-toggle-button
           pref="{{prefs.download.prompt_for_download}}"
           label="$i18n{promptForDownload}">
       </settings-toggle-button>
 <if expr="chromeos">
       <settings-toggle-button
           pref="{{prefs.gdata.disabled}}"
           label="$i18n{disconnectGoogleDriveAccount}"
           hidden="[[!pageVisibility.googleDrive]]">
       </settings-toggle-button>
 </if>
     </div>
   </template>
   <script src="downloads_page.js"></script>
 </dom-module>