MD Settings: made proxy button respect policy

chrome/browser/resources/settings/system_page/system_page.html

 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-button/paper-button.html">
+<link rel="import" href="/controls/controlled_button.html">
 <link rel="import" href="/controls/settings_checkbox.html">
 <link rel="import" href="/lifetime_browser_proxy.html">
 <link rel="import" href="/prefs/prefs.html">
 <link rel="import" href="/settings_shared_css.html">
 <link rel="import" href="/system_page/system_page_browser_proxy.html">
 
 <dom-module id="settings-system-page">
   <template>
     <style include="settings-shared">
       /* TODO(dbeam): is it worth generalizing this style? Does something like
@@ skipping 17 matching lines @@
         <settings-checkbox label="$i18n{hardwareAccelerationLabel}"
             pref="{{prefs.hardware_acceleration_mode.enabled}}">
         </settings-checkbox>
         <template is="dom-if" if="[[shouldShowRestart_(
             prefs.hardware_acceleration_mode.enabled.value)]]">
           <paper-button on-tap="onRestartTap_">$i18n{restart}</paper-button>
         </template>
       </div>
     </div>
     <div class="settings-box">
-      <paper-button class="primary-button" on-tap="onChangeProxySettingsTap_">
+      <controlled-button class="primary-button" pref="[[prefs.proxy]]"
+          on-tap="onChangeProxySettingsTap_">
         $i18n{changeProxySettings}
-      </paper-button>
+      </controlled-button>

[stevenjb, 2016/07/27 16:47:32]

This is *so* much better than what we have in options. Thanks!

[Dan Beam, 2016/07/29 03:46:51]

just imagine how great everything will be the THIRD time we write settings!

     </div>
   </template>
   <script src="/system_page/system_page.js"></script>
 </dom-module>