MD Settings: add a <controlled-radio-button>

chrome/browser/resources/settings/about_page/channel_switcher_dialog.html

 <link rel="import" href="chrome://resources/cr_elements/cr_dialog/cr_dialog.html">
 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/html/i18n_behavior.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-button/paper-button.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-radio-button/paper-radio-button.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-radio-group/paper-radio-group.html">
 <link rel="import" href="/about_page/about_page_browser_proxy.html">
 <link rel="import" href="/settings_shared_css.html">
 
 <dom-module id="settings-channel-switcher-dialog">
   <template>
     <style include="settings-shared"></style>
     <dialog is="cr-dialog" id="dialog">
       <div class="title">$i18n{aboutChangeChannel}</div>
       <div class="body">
+        <!-- TODO(dbeam): this can be policy-controlled. Show this in the UI.
+             https://www.chromium.org/administrators/policy-list-3#ChromeOsReleaseChannel
+        -->
         <paper-radio-group
             on-paper-radio-group-changed="onChannelSelectionChanged_">
           <paper-radio-button name="[[browserChannelEnum_.STABLE]]">
             $i18n{aboutChannelStable}
           </paper-radio-button>
           <paper-radio-button name="[[browserChannelEnum_.BETA]]">
             $i18n{aboutChannelBeta}
           </paper-radio-button>
           <paper-radio-button name="[[browserChannelEnum_.DEV]]">
             $i18n{aboutChannelDev}
@@ skipping 15 matching lines @@
         <paper-button id="changeChannelAndPowerwash" class="action-button"
             on-tap="onChangeChannelAndPowerwashTap_"
             hidden="[[!shouldShowButtons_.changeChannelAndPowerwash]]">
           $i18n{aboutChangeChannelAndPowerwash}
         </paper-button>
       </div>
     </dialog>
   </template>
   <script src="channel_switcher_dialog.js"></script>
 </dom-module>