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

chrome/browser/resources/settings/settings_vars_css.html

 <link rel="import" href="chrome://resources/cr_elements/shared_vars_css.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-styles/color.html">
 
 <!-- Common css variables for Material Design settings. -->
 <style is="custom-style">
   /* We keep our vars in sort order, though some vars must be defined prior to
    * others. The --settings-* vars are defined explicitly prior to the --paper-*
    * and --iron-* vars.
    */
   :root {
@@ skipping 64 matching lines @@
     --settings-toggle-ink-size: {
       height: 40px;
       top: -12px;
       left: -12px;
       width: 40px;
     };
     --settings-toggle-color: var(--google-blue-500);
 
     --checkbox-margin-start: 2px;
     --checkbox-size: 16px;
-    --checkbox-spacing: 18px;
+    --settings-control-spacing: 18px;
     --iron-icon-fill-color: var(--paper-grey-600);
     --iron-icon-height: var(--cr-icon-size);
     --iron-icon-width: var(--cr-icon-size);
     --paper-checkbox-label-color: inherit;
     --paper-dialog-color: inherit;
 
     --cr-icon-ripple-size: 40px;
     --paper-icon-button: {
       /**
        * This makes the icons 20px (in combination with "--cr-icon-ripple-size:
@@ skipping 42 matching lines @@
     --paper-toggle-button-unchecked-button: var(--settings-toggle-button-size);
     --paper-toggle-button-unchecked-ink: var(--settings-toggle-ink-size);
 
     --settings-input-underline: {
       border-color: var(--paper-grey-300);
     };
 
     --paper-input-container-underline: var(--settings-input-underline);
   }
 </style>