[MD settings] tweaks to what is shown in about page; and css cleanup

chrome/browser/resources/settings/people_page/setup_pin_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="/people_page/pin_keyboard.html">
 <link rel="import" href="/settings_shared_css.html">
 
 <dom-module id="settings-setup-pin-dialog">
   <template>
     <style include="settings-shared"></style>
     <style>
-      iron-icon {
-        -webkit-margin-end: var(--iron-icon-spacing);
-      }
-
       .warning {
         color: var(--paper-grey-500);
       }
 
       .warning > iron-icon {
         --iron-icon-fill-color: var(--paper-grey-500);
       }
 
       .error {
         color: var(--paper-red-500);
       }
 
       .error > iron-icon {
         --iron-icon-fill-color: var(--paper-red-500);
       }
 
       .align-center {
         justify-content: center;
       }
     </style>
 
     <dialog is="cr-dialog" id="dialog" on-close="close">
       <div class="title">[[getTitleMessage_(isConfirmStep_)]]</div>
       <div class="body">
         <!-- Warning/error; only shown if title is hidden. -->
         <div class$="[[problemClass_]] settings-box first"
              hidden$="[[!hasProblem_(problemMessage_)]]">
           <iron-icon icon="icons:warning"></iron-icon>
-          <span>[[problemMessage_]]</span>
+          <span class="middle">[[problemMessage_]]</span>
         </div>
 
         <!-- Pin keyboard -->
         <div class="align-center settings-box continuation">
           <pin-keyboard id="pinKeyboard" on-pin-change="onPinChange_"
                         on-submit="onPinSubmit_" value="{{pinKeyboardValue_}}">
           </pin-keyboard>
         </div>
 
         <div class="button-strip">
           <paper-button class="cancel-button" on-tap="onCancelTap_">
             $i18n{cancel}
           </paper-button>
 
           <paper-button class="action-button" on-tap="onPinSubmit_"
                         disabled$="[[!enableSubmit_]]">
             <span>[[getContinueMessage_(isConfirmStep_)]]</span>
           </paper-button>
         </div>
       </div>
     </dialog>
   </template>
 
   <script src="setup_pin_dialog.js"></script>
 </dom-module>