WebUI: Unify [hidden] attribute styling rules.

ui/webui/resources/cr_elements/hidden_style_css.html

Index: ui/webui/resources/cr_elements/hidden_style_css.html
diff --git a/ui/webui/resources/cr_elements/hidden_style_css.html b/ui/webui/resources/cr_elements/hidden_style_css.html
new file mode 100644
index 0000000000000000000000000000000000000000..4c22288edac7ea8a8adabe979adb3ef7b9cf92ff
--- /dev/null
+++ b/ui/webui/resources/cr_elements/hidden_style_css.html
@@ -0,0 +1,15 @@
+<link rel="import" href="chrome://resources/html/polymer.html">
+
+<!-- Common style for Material Design WebUI, such that the |hidden| attributes
+  works within Shadow DOM. -->
+<dom-module id="cr-hidden-style">
+  <template>
+    <style>
+      /* Included here so we don't have to include "iron-positioning" in every
+       * stylesheet. See crbug.com/498405. */
+      [hidden] {
+        display: none !important;
+      }
+    </style>
+  </template>
+</dom-module>