MDUserManager: import the appropriate dom-modules form iron-flex-layout-classes

chrome/browser/resources/md_user_manager/supervised_user_learn_more.html

 <link rel="import" href="/icons.html">
 <link rel="import" href="/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-classes.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 iron-flex iron-flex-alignment">
+    <style include="shared-styles iron-flex iron-flex-alignment iron-positioning">
       :host {
         align-self: center;
       }
 
       #container {
         color: var(--primary-text-color);
         line-height: 20px;
         width: var(--page-width);
       }
 
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       </div>
       <div id="actions">
         <paper-button id="done" class="action primary" on-tap="onDoneTap_">
           $i18n{supervisedUserLearnMoreDone}
         </paper-button>
       </div>
     </div>
   </template>
   <script src="supervised_user_learn_more.js"></script>
 </dom-module>