MD Settings: Add missing cr-hidden-style in settings_subpage_search.html.

BUG=715906
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2846103003
Cr-Commit-Position: refs/heads/master@{#467856}

chrome/browser/resources/settings/settings_page/settings_subpage_search.html

 <link rel="import" href="chrome://resources/cr_elements/cr_search_field/cr_search_field_behavior.html">
+<link rel="import" href="chrome://resources/cr_elements/hidden_style_css.html">
 <link rel="import" href="chrome://resources/cr_elements/icons.html">
 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-icon/iron-icon.html">
@@ -7,7 +8,7 @@
 
 <dom-module id="settings-subpage-search">
   <template>
-    <style>
+    <style include="cr-hidden-style">
       :host {
         --subpage-search-underline: {
           margin-bottom: -2px;