WebUI: Unify [hidden] attribute styling rules.

chrome/browser/resources/settings/settings_shared_css.html

 <link rel="import" href="chrome://resources/cr_elements/shared_style_css.html">
 <link rel="import" href="settings_vars_css.html">
 
 <!-- Common styles for Material Design settings. -->
 <dom-module id="settings-shared">
   <template>
     <style include="cr-shared-style">
-      /* Included here so we don't have to include "iron-positioning" in every
-       * stylesheet. See crbug.com/498405. */
-      [hidden] {
-        display: none !important;
-      }
-
       :host-context([dir=rtl]) button[is='paper-icon-button-light'] {
         transform: scaleX(-1);  /* Flip on the X axis (aka mirror). */
       }
 
       :host-context([dir=rtl]) paper-icon-button[icon='settings:arrow-back'] {
         transform: scaleX(-1);  /* Flip on the X axis (aka mirror). */
       }
 
       /* Use <h2> as the "sub-header" mentioned in the UX design docs. */
       h2 {
@@ skipping 371 matching lines @@
         transform: rotate(-135deg);
       }
 
       .column-header {
         color: var(--paper-grey-600);
         font-weight: 500;
       }
     </style>
   </template>
 </dom-module>