MD Settings: Tweak margin of "Trust settings" header.

chrome/browser/resources/settings/certificate_manager_page/ca_trust_edit_dialog.html

 <link rel="import" href="chrome://resources/cr_elements/cr_dialog/cr_dialog.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="chrome://resources/polymer/v1_0/paper-checkbox/paper-checkbox.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-spinner/paper-spinner.html">
 <link rel="import" href="../i18n_setup.html">
 <link rel="import" href="../settings_shared_css.html">
 <link rel="import" href="certificates_browser_proxy.html">
 
 <dom-module id="settings-ca-trust-edit-dialog">
   <template>
     <style include="settings-shared">
       paper-checkbox {
         display: block;
+      }
+
+      paper-checkbox,
+      #description {
         margin: 15px 0;
       }
     </style>
 
     <dialog is="cr-dialog" id="dialog" close-text="$i18n{close}">
       <div class="title">$i18n{certificateManagerCaTrustEditDialogTitle}</div>
       <div class="body">
         <div>[[explanationText_]]</div>
-        <div>$i18n{certificateManagerCaTrustEditDialogDescription}</div>
+        <div id="description">
+          $i18n{certificateManagerCaTrustEditDialogDescription}
+        </div>
         <paper-checkbox id="ssl" checked="[[trustInfo_.ssl]]">
           $i18n{certificateManagerCaTrustEditDialogSsl}
         </paper-checkbox>
         <paper-checkbox id="email" checked="[[trustInfo_.email]]">
           $i18n{certificateManagerCaTrustEditDialogEmail}
         </paper-checkbox>
         <paper-checkbox id="objSign" checked="[[trustInfo_.objSign]]">
           $i18n{certificateManagerCaTrustEditDialogObjSign}
         </paper-checkbox>
       </div>
       <div class="button-container">
         <paper-spinner id="spinner"></paper-spinner>
         <paper-button class="cancel-button" on-tap="onCancelTap_">
           $i18n{cancel}
         </paper-button>
         <paper-button id="ok" class="action-button" on-tap="onOkTap_">
           $i18n{ok}
         </paper-button>
       </div>
     </dialog>
   </template>
   <script src="ca_trust_edit_dialog.js"></script>
 </dom-module>