[MD settings] CSS var --checkbox-spacing to --settings-control-spacing

chrome/browser/resources/settings/controls/settings_dropdown_menu.html

 <link rel="import" href="chrome://resources/html/md_select_css.html">
 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/cr_elements/policy/cr_policy_pref_behavior.html">
 <link rel="import" href="chrome://resources/cr_elements/policy/cr_policy_pref_indicator.html">
 <link rel="import" href="pref_control_behavior.html">
 <link rel="import" href="../i18n_setup.html">
 <link rel="import" href="../prefs/pref_util.html">
 <link rel="import" href="../settings_shared_css.html">
 <link rel="import" href="../settings_vars_css.html">
 
 <dom-module id="settings-dropdown-menu">
   <template>
     <style include="settings-shared md-select">
       :host {
         display: inline-flex;
       }
 
       cr-policy-pref-indicator {
-        margin: 0 var(--checkbox-spacing);
+        margin: 0 var(--settings-control-spacing);
       }
 
       /* Hide "Custom" value when unselectable. */
       option:disabled {
         display: none;
       }
     </style>
     <template is="dom-if" if="[[pref.controlledBy]]" restamp>
       <cr-policy-pref-indicator pref="[[pref]]"></cr-policy-pref-indicator>
     </template>
     <div class="md-select-wrapper">
       <select class="md-select" id="dropdownMenu" on-change="onChange_"
           disabled="[[shouldDisableMenu_(disabled, menuOptions, pref.*)]]">
         <template is="dom-repeat" items="[[menuOptions]]">
           <option value="[[item.value]]">[[item.name]]</option>
         </template>
         <option value="[[notFoundValue_]]"
             disabled="[[!showNotFoundValue_(menuOptions, pref.value)]]">
           $i18n{custom}
         </option>
       </select>
       <span class="md-select-underline"></span>
     </div>
   </template>
   <script src="settings_dropdown_menu.js"></script>
 </dom-module>