[MD settings] small code health changes

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

 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-icon/iron-icon.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-resizable-behavior/iron-resizable-behavior.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/neon-animation/neon-animatable-behavior.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-icon-button/paper-icon-button.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-ripple/paper-ripple.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-spinner/paper-spinner.html">
 <link rel="import" href="../icons.html">
 <link rel="import" href="../route.html">
 <link rel="import" href="settings_subpage_search.html">
 <link rel="import" href="../settings_shared_css.html">
 
 <dom-module id="settings-subpage">
   <template>
     <style include="settings-shared">
       :host {
         display: block;
         padding-bottom: 60px;
       }
 
-      .settings-box.first {
+      #headerLine {
         min-height: 40px;
         padding-top: 8px;
       }
 
-      .settings-box.first > h1 {
+      #headerLine > h1 {
         -webkit-margin-start: 4px;
       }
 
       #learnMore {
         @apply(--cr-paper-icon-button-margin);
         align-items: center;
         display: flex;
         height: var(--cr-icon-ripple-size);
         justify-content: center;
         position: relative;  /* Needed for paper-ripple. */
@@ skipping 15 matching lines @@
         flex: 1;  /* Push other items to the end. */
         font-size: 107.6923%;  /* Go to 14px from 13px */
         font-weight: 500;
       }
 
       settings-subpage-search {
         /* Keep normal icon spacing from subpage-title-extra controls. */
         -webkit-margin-start: 16px;
       }
     </style>
-    <div class="settings-box first">
+    <div class="settings-box first" id="headerLine">
       <paper-icon-button icon="settings:arrow-back" on-tap="onTapBack_"
           aria-label="$i18n{back}">
       </paper-icon-button>
       <h1>[[pageTitle]]</h1>
       <content select=".subpage-title-extra"></content>
       <template is="dom-if" if="[[learnMoreUrl]]">
         <a id="learnMore" aria-label="$i18n{learnMore}"
             href="[[learnMoreUrl]]" target="_blank">
           <iron-icon icon="settings:help-outline"></iron-icon>
           <paper-ripple class="circle" center></paper-ripple>
         </a>
       </template>
       <template is="dom-if" if="[[searchLabel]]">
         <settings-subpage-search label="[[searchLabel]]"
             on-search-changed="onSearchChanged_">
         </settings-subpage-search>
       </template>
       <template is="dom-if" if="[[showSpinner]]">
         <paper-spinner active></paper-spinner>
       </template>
     </div>
     <content></content>
   </template>
   <script src="settings_subpage.js"></script>
 </dom-module>