[MD History] Move background-clip div behind items.

chrome/browser/resources/md_history/history_item.html

 <link rel="import" href="chrome://resources/cr_elements/icons.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/paper-button/paper-button.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-checkbox/paper-checkbox.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-icon-button/paper-icon-button-light.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-styles/shadow.html">
 <link rel="import" href="chrome://resources/html/icon.html">
 <link rel="import" href="chrome://resources/html/util.html">
 <link rel="import" href="chrome://history/browser_service.html">
@@ skipping 107 matching lines @@
       }
 
       #background-clip {
         /* Prevent artifacts when zoomed by overlapping the next item. */
         bottom: -0.4px;
         clip: rect(auto 999px auto -5px); /* Clip the top and bottom edges. */
         left: 0;
         position: absolute;
         right: 0;
         top: 0;
+        z-index: -1;
       }
 
       :host([is-card-end]) #background-clip {
         bottom: 0;
         clip: rect(auto 999px 500px -5px); /* Clip the top edge. */
       }
 
       #background {
         @apply(--shadow-elevation-2dp);
         background: #fff;
         bottom: 0;
         left: 0;
         position: absolute;
         right: 0;
         top: 0;
-        z-index: -1;
       }
 
       :host([embedded]) #background {
         display: none;
       }
 
       :host(:not([is-card-start])) #background {
         top: -5px; /* Draw the box shadow up the full edge of the background. */
       }
 
@@ skipping 50 matching lines @@
             <div></div>
             <div></div>
           </button>
         </div>
         <div id="time-gap-separator" hidden="[[!hasTimeGap]]"></div>
       </div>
     </div>
   </template>
   <script src="chrome://history/history_item.js"></script>
 </dom-module>