WebUI: Unify [hidden] attribute styling rules.

chrome/browser/resources/md_downloads/manager.html

+<link rel="import" href="chrome://resources/cr_elements/hidden_style_css.html">
 <link rel="import" href="chrome://resources/html/cr.html">
 <link rel="import" href="chrome://resources/html/cr/ui.html">
 <link rel="import" href="chrome://resources/html/cr/ui/command.html">
 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/html/util.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-a11y-announcer/iron-a11y-announcer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-list/iron-list.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/item.html">
 <link rel="import" href="chrome://downloads/toolbar.html">
 
 <dom-module id="downloads-manager">
   <template>
-    <style>
+    <style include="cr-hidden-style">
       :host {
         display: flex;
         flex: 1 0;
         flex-direction: column;
         height: 100%;
         z-index: 0;
       }
 
-      [hidden] {
-        display: none !important;
-      }
-
       @media screen and (max-width: 1024px) {
         :host {
           flex-basis: calc(
               var(--downloads-card-width) + 2 * var(--downloads-card-margin));
         }
       }
 
       #toolbar {
         position: relative;
         z-index: 1;
@@ skipping 65 matching lines @@
     </iron-list>
     <div id="no-downloads" hidden="[[hasDownloads_]]">
       <div>
         <div class="illustration"></div>
         <span>[[noDownloadsText_(inSearchMode_)]]</span>
       </div>
     </div>
   </template>
   <script src="chrome://downloads/manager.js"></script>
 </dom-module>