MD User Manager: Refactors existing dialogs in User Manager into <user-manager-dialog>

chrome/browser/resources/md_user_manager/supervised_user_learn_more.html

 <link rel="import" href="chrome://md-user-manager/icons.html">
 <link rel="import" href="chrome://md-user-manager/shared_styles.html">
 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-flex-layout/iron-flex-layout.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-button/paper-button.html">
 
 <dom-module id="supervised-user-learn-more">
   <template>
     <style include="shared-styles">
       :host {
         align-self: center;
       }
 
       #container {
         color: var(--primary-text-color);
         line-height: 20px;
         width: var(--page-width);
       }
 
       #title-area {
-        border-bottom: 1px solid rgba(0, 0, 0, .12);
+        border-bottom: var(--user-manager-separator-line);
         font-size: 16px;
         padding-bottom: 16px;
       }
 
       #title-area iron-icon {
         --iron-icon-height: 20px;
         --iron-icon-width: 20px;
         color: var(--title-icon-color);
       }
 
@@ skipping 24 matching lines @@
       </div>
       <div id="actions">
         <paper-button id="done" class="action primary" on-tap="onDoneTap_">
           $i18n{supervisedUserLearnMoreDone}
         </paper-button>
       </div>
     </div>
   </template>
   <script src="chrome://md-user-manager/supervised_user_learn_more.js"></script>
 </dom-module>