Smart Lock: Set an associated-control on the settings subpage.

The associated-control allows the top-level Settings UI to display an arrow under a control (e.g. a button) whose subpage matches the search text.

R=hansberry@chromium.org

Bug: 901381
Test: none
Change-Id: I0e4f4c3b8939213853273a8b186ae082c5bacbed
Reviewed-on: https://chromium-review.googlesource.com/c/1319336Reviewed-by: Ryan Hansberry <hansberry@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: James Hawkins <jhawkins@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605805}

chrome/browser/resources/settings/multidevice_page/multidevice_page.html

@@ -99,7 +99,9 @@
       </template>
       <template is="dom-if" route-path="/multidevice/features/smartLock"
           restamp>
-        <settings-subpage page-title="$i18n{easyUnlockSectionTitle}">
+        <settings-subpage
+            associated-control="[[$$('#multidevice-item')]]"
+            page-title="$i18n{easyUnlockSectionTitle}">
           <settings-multidevice-smartlock-subpage
               prefs="{{prefs}}"
               page-content-data="[[pageContentData]]">