[MD-Settings] Make section titles into <h2> headings for a11y.

chrome/browser/resources/settings/settings_page/settings_section.html

 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-styles/shadow.html">
 <link rel="import" href="/animation/animation.html">
 
 <dom-module id="settings-section">
   <template>
     <style>
       :host {
         display: flex;
         flex-direction: column;
         /* Margin so the box-shadow isn't clipped during animations. */
         margin-left: 3px;
         margin-right: 3px;
         position: relative;
       }
 
       #header {
         margin-bottom: 12px;
       }
 
       #header .title {
         color: var(--paper-grey-600);
         font-size: 13px;
         font-weight: 500;
+        margin-bottom: 0;
         margin-top: var(--settings-page-vertical-margin);
       }
 
       #card {
         @apply(--shadow-elevation-2dp);
         background-color: white;
         flex: 1;
         overflow: hidden;
       }
 
@@ skipping 12 matching lines @@
 
       :host(.expanded) #header {
         display: none;
       }
 
       :host([hidden-by-search]) {
         display: none;
       }
     </style>
     <div id="header">
-      <div class="title">{{pageTitle}}</div>
+      <h2 class="title">{{pageTitle}}</h2>
     </div>
     <div id="card">
       <content id="content"></content>
     </div>
   </template>
   <script src="settings_section.js"></script>
 </dom-module>