MD Settings: Expanding cards should not overlap header, take 2.

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

 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://md-settings/settings_page/transition_behavior.html">
 <script src="chrome://md-settings/settings_page/main_page_behavior.js"></script>
 
 <dom-module id="main-page-styles">
   <template>
     <style>
       .expanding,
       .collapsing {
-        z-index: 2;
+        /* Must be lower than the paper-header-panel's z-index.
+         * See settings_ui.html. */
+        z-index: 1;
       }
 
       .expanded {
         min-height: 100%;
       }
 
       .expanded.frozen {
         /* Account for padding in settings_page.css. */
         margin: -24px 0;
         min-height: calc(100% + 48px);
       }
     </style>
   </template>
 </dom-module>