WebUI: Unify [hidden] attribute styling rules.

chrome/browser/resources/settings/settings_shared_css.html

Index: chrome/browser/resources/settings/settings_shared_css.html
diff --git a/chrome/browser/resources/settings/settings_shared_css.html b/chrome/browser/resources/settings/settings_shared_css.html
index 5204d00657ace2e0ebd7cd40148354e81fa2a4e5..cfcd2dd8784344b9389c2bb4c93cb905271ecbb5 100644
--- a/chrome/browser/resources/settings/settings_shared_css.html
+++ b/chrome/browser/resources/settings/settings_shared_css.html
@@ -5,12 +5,6 @@
 <dom-module id="settings-shared">
   <template>
     <style include="cr-shared-style">
-      /* Included here so we don't have to include "iron-positioning" in every
-       * stylesheet. See crbug.com/498405. */
-      [hidden] {
-        display: none !important;
-      }
-
       :host-context([dir=rtl]) button[is='paper-icon-button-light'] {
         transform: scaleX(-1);  /* Flip on the X axis (aka mirror). */
       }