MD Settings: make "CHANGE PROXY SETTINGS" button primary styled

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

 <link rel="import" href="chrome://resources/polymer/v1_0/polymer/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-button/paper-button.html">
 <link rel="import" href="chrome://md-settings/controls/settings_checkbox.html">
 <link rel="import" href="chrome://md-settings/settings_shared_css.html">
 
 <dom-module id="settings-system-page">
   <template>
     <style include="settings-shared"></style>
     <div class="settings-box block first">
 <if expr="not is_macosx">
       <settings-checkbox i18n-values="label:backgroundAppsLabel"
           pref="{{prefs.background_mode.enabled}}">
       </settings-checkbox>
 </if>
       <settings-checkbox i18n-values="label:hardwareAccelerationLabel"
           pref="{{prefs.hardware_acceleration_mode.enabled}}">
         <!-- TODO(dbeam): restart button. -->
       </settings-checkbox>
     </div>
     <div class="settings-box">
       <div class="button-strip">
-        <paper-button i18n-content="changeProxySettings"
+        <paper-button i18n-content="changeProxySettings" class="primary-button"
             on-tap="onChangeProxySettingsTap_"></paper-button>
       </div>
     </div>
   </template>
   <script src="chrome://md-settings/system_page/system_page.js"></script>
 </dom-module>