Add pref to enable/disable palette tray.

chrome/browser/resources/settings/device_page/stylus.html

 <link rel="import" href="chrome://resources/html/action_link.html">
 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="/controls/settings_checkbox.html">
 <link rel="import" href="/settings_shared_css.html">
 
 <dom-module id="settings-stylus">
   <template>
     <style include="settings-shared"></style>
 
     <div class="settings-box">
       <settings-checkbox
+          pref="{{prefs.settings.enable_stylus_tools}}"
+          label="$i18n{stylusEnableStylusTools}">
+      </settings-checkbox>
+    </div>
+
+    <div class="settings-box continuation">
+      <settings-checkbox
           pref="{{prefs.settings.launch_palette_on_eject_event}}"
           label="$i18n{stylusAutoOpenStylusTools}">
       </settings-checkbox>
     </div>
 
     <div class="settings-box">
       <!-- TODO(jdufault): This should launch an intent (using the
         enable-intent-picker flag) that opens up the play store to a curated
         list of apps. -->
       <a is="action-link" target="_blank"
          href="https://play.google.com/store/apps/collection/promotion_30023cb_stylus_apps">
         $i18n{stylusFindMoreApps}
       </a>
     </div>
 
   </template>
   <script src="stylus.js"></script>
 </dom-module>