WebUI: Unify [hidden] attribute styling rules.

chrome/browser/resources/md_bookmarks/shared_style.html

Index: chrome/browser/resources/md_bookmarks/shared_style.html
diff --git a/chrome/browser/resources/md_bookmarks/shared_style.html b/chrome/browser/resources/md_bookmarks/shared_style.html
index f242649702e004e33e4a7031d7a614676d3f04a7..c8d75709a50d82b7d80ae359d1502a12c997d36c 100644
--- a/chrome/browser/resources/md_bookmarks/shared_style.html
+++ b/chrome/browser/resources/md_bookmarks/shared_style.html
@@ -1,12 +1,9 @@
 <link rel="import" href="chrome://bookmarks/shared_vars.html">
+<link rel="import" href="chrome://resources/cr_elements/hidden_style_css.html">
 
 <dom-module id="shared-style">
   <template>
-    <style>
-      [hidden] {
-        display: none !important;
-      }
-
+    <style include="cr-hidden-style">
       button.more-vert-button {
         height: 36px;
         padding: 8px;