[MD settings] refine routing function

chrome/browser/resources/settings/settings_page_css.html

 <link rel="import" href="/settings_page/main_page_behavior.html">
 
 <dom-module id="settings-page-styles">
   <template>
     <style include="main-page-styles">
       :host {
         --settings-page-padding-top: 21px;
         box-sizing: border-box;
         display: block;
-        height: 100%;
+        height: inherit;
         margin: 0 auto;
         max-width: 960px;
         min-width: 622px;
         position: relative;
         width: 96%;
       }
 
-      :host > * {
+      :host > div {
+        height: inherit;
+      }
+
+      :host > div > * {
         /* The margin and padding here are doing two things: make the total
          * separation 24px; and make scrollIntoView align the section header
          * with the top item in the side nav menu. Both things are desired
          * by Alan (bettes@). */
         margin-bottom: 3px;
         padding-top: var(--settings-page-padding-top);
       }
 
       .expanded.frozen {
         margin-top: -var(--settings-page-padding-top);
       }
     </style>
   </template>
 </dom-module>