WebUI: Unify [hidden] attribute styling rules.

chrome/browser/resources/md_downloads/toolbar.html

Index: chrome/browser/resources/md_downloads/toolbar.html
diff --git a/chrome/browser/resources/md_downloads/toolbar.html b/chrome/browser/resources/md_downloads/toolbar.html
index bda557e498fd7927a5f5fbb35d09fab6613af5a2..2b6bca652e8d0e0e49dab92d9fafdab6a3342e03 100644
--- a/chrome/browser/resources/md_downloads/toolbar.html
+++ b/chrome/browser/resources/md_downloads/toolbar.html
@@ -1,4 +1,5 @@
 <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">
@@ -12,7 +13,7 @@
 
 <dom-module id="downloads-toolbar">
   <template>
-    <style>
+    <style include="cr-hidden-style">
       :host {
         align-items: center;
         background: var(--google-blue-700);
@@ -21,10 +22,6 @@
         min-height: 56px;
       }
 
-      [hidden] {
-        display: none !important;
-      }
-
       #toolbar {
         --cr-toolbar-field-width: var(--downloads-card-width);
         flex: 1;