WebUI: Unify [hidden] attribute styling rules.

chrome/browser/resources/md_downloads/toolbar.html

 <link rel="import" href="chrome://downloads/action_service.html">
+<link rel="import" href="chrome://resources/cr_elements/hidden_style_css.html">
 <link rel="import" href="chrome://resources/html/assert.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/html/util.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-icon-button/paper-icon-button-light.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-styles/color.html">
 <link rel="import" href="chrome://resources/cr_elements/cr_action_menu/cr_action_menu.html">
 <link rel="import" href="chrome://resources/cr_elements/cr_toolbar/cr_toolbar.html">
 <link rel="import" href="chrome://resources/cr_elements/icons.html">
 
 <dom-module id="downloads-toolbar">
   <template>
-    <style>
+    <style include="cr-hidden-style">
       :host {
         align-items: center;
         background: var(--google-blue-700);
         color: white;
         display: flex;
         min-height: 56px;
       }
 
-      [hidden] {
-        display: none !important;
-      }
-
       #toolbar {
         --cr-toolbar-field-width: var(--downloads-card-width);
         flex: 1;
       }
 
       #moreActions {
         --iron-icon-height: 20px;
         --iron-icon-width: 20px;
         height: 32px;
         padding: 6px;
@@ skipping 16 matching lines @@
         $i18n{clearAll}
       </div>
       <button class="dropdown-item" on-tap="onOpenDownloadsFolderTap_"
           role="menuitem">
         $i18n{openDownloadsFolder}
       </button>
     </dialog>
   </template>
   <script src="chrome://downloads/toolbar.js"></script>
 </dom-module>