[MD settings] using h2 for sub-headers consistently.

chrome/browser/resources/settings/passwords_and_forms_page/passwords_shared_css.html

 <!-- Common styles for Passwords and Forms -->
 <dom-module id="passwords-shared">
   <template>
     <style>
       :host {
         -webkit-margin-end: 20px;
         -webkit-margin-start: 20px;
         display: flex;
         flex-direction: column;
       }
 
       .column-header {
         color: var(--google-grey-500);
         font-weight: 500;
       }
 
-      .heading {
-        margin-bottom: 8px;
-      }
-
       paper-icon-button {
         -webkit-margin-end: 0;
         -webkit-margin-start: 20px;
         -webkit-padding-end: 0;
         -webkit-padding-start: 0;
         color: var(--paper-grey-600);
         width: 20px;
       }
 
       .item-list {
         -webkit-margin-start: 36px;
         margin-bottom: 16px;
       }
 
       .list-with-header > div:first-of-type {
         border-top: var(--settings-separator-line);
       }
 
       /* TODO(hcarmona): Grow menu width by 64px if content is wider */
       .menu-item {
         -webkit-padding-start: 24px;
         background-color: white;
         border: none;
         font: inherit;
         width: 104px;
       }
     </style>
   </template>
 </dom-module>