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

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

 <link rel="import" href="chrome://resources/cr_elements/policy/cr_policy_pref_indicator.html">
 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-checkbox/paper-checkbox.html">
 <link rel="import" href="settings_boolean_control_behavior.html">
 <link rel="import" href="../settings_shared_css.html">
 
 <dom-module id="settings-checkbox">
   <template>
     <style include="settings-shared">
       #outerRow {
         align-items: center;
         display: flex;
         min-height: var(--settings-row-two-line-min-height);
         width: 100%;
       }
 
       #outerRow[noSubLabel] {
         min-height: var(--settings-row-min-height);
       }
 
       paper-checkbox {
         width: 100%;
       }
 
       paper-checkbox:not([checked]) .secondary {
         @apply(--settings-secondary-unchecked);
       }
 
       cr-policy-pref-indicator {
-        -webkit-margin-start: var(--checkbox-spacing);
+        -webkit-margin-start: var(--settings-control-spacing);
       }
     </style>
     <div id="outerRow" noSubLabel$="[[!subLabel]]">
       <paper-checkbox id="checkbox" checked="{{checked}}"
           on-change="notifyChangedByUserInteraction"
           disabled="[[controlDisabled_(disabled, pref.*)]]">
         <div>[[label]] <content></content></div>
         <div class="secondary">[[subLabel]]</div>
       </paper-checkbox>
       <template is="dom-if" if="[[pref.controlledBy]]">
         <cr-policy-pref-indicator pref="[[pref]]" icon-aria-label="[[label]]">
         </cr-policy-pref-indicator>
       </template>
     </div>
   </template>
   <script src="settings_checkbox.js"></script>
 </dom-module>