WebUI: Unify [hidden] attribute styling rules.

chrome/browser/resources/md_downloads/item.html

Index: chrome/browser/resources/md_downloads/item.html
diff --git a/chrome/browser/resources/md_downloads/item.html b/chrome/browser/resources/md_downloads/item.html
index f0577352179bd10d0ee49d854d6d753f6b8baaa1..c4ed060ace7dd8a9e40a9818f2e00337d0731c72 100644
--- a/chrome/browser/resources/md_downloads/item.html
+++ b/chrome/browser/resources/md_downloads/item.html
@@ -1,4 +1,5 @@
 <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">
@@ -16,16 +17,12 @@
 
 <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;