WebUI: Unify [hidden] attribute styling rules.

chrome/browser/resources/md_downloads/item.html

 <link rel="import" href="chrome://resources/cr_elements/icons.html">
+<link rel="import" href="chrome://resources/cr_elements/hidden_style_css.html">
 <link rel="import" href="chrome://resources/html/action_link.html">
 <link rel="import" href="chrome://resources/html/action_link_css.html">
 <link rel="import" href="chrome://resources/html/cr.html">
 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-icon/iron-icon.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-icon-button/paper-icon-button-light.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-progress/paper-progress.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-styles/color.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-styles/shadow.html">
 <link rel="import" href="chrome://downloads/action_service.html">
 <link rel="import" href="chrome://downloads/constants.html">
 <link rel="import" href="chrome://downloads/i18n_setup.html">
 <link rel="import" href="chrome://downloads/icons.html">
 
 <dom-module id="downloads-item">
   <template>
-    <style include="action-link">
+    <style include="action-link cr-hidden-style">
       :host {
         display: flex;
         flex-direction: column;
       }
 
-      [hidden] {
-        display: none !important;
-      }
-
       paper-button {
         font-weight: 500;
         margin: 0;
         min-width: auto;
       }
 
       #date {
         color: var(--paper-grey-700);
         font-size: 100%;
         font-weight: 500;
@@ skipping 274 matching lines @@
             on-tap="onRemoveTap_">&#x2715;</button>
       </div>
 
       <div id="incognito" title="$i18n{inIncognito}" hidden="[[!data.otr]]">
       </div>
     </div>
 
   </template>
   <script src="chrome://downloads/item.js"></script>
 </dom-module>