[MD History] Use box-shadow to render card shadows everywhere.

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">
 <link rel="import" href="chrome://history/constants.html">
 <link rel="import" href="chrome://history/searched_label.html">
 <link rel="import" href="chrome://history/shared_style.html">
 
 <dom-module id="history-item">
   <template>
     <style include="shared-style">
@@ skipping 92 matching lines @@
         height: 16px;
         width: 16px;
       }
 
       #time-gap-separator {
         -webkit-border-start: 1px solid #888;
         -webkit-margin-start: 77px;
         height: 15px;
       }
 
-      #background {
-        background: #fff;
-        bottom: -1px;  /* Prevents shadow artifacts when zoomed */
+      #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;
+      }
+
+      :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. */
+      }
+
       :host([is-card-start]) #background {
         border-radius: 2px 2px 0 0;
       }
 
       :host([is-card-end]) #background {
         border-radius: 0 0 2px 2px;
-        bottom: 0;
+      }
+
+      :host([is-card-start][is-card-end]) #background {
+        border-radius: 2px;
       }
     </style>
 
     <div id="sizing-container">
       <div id="main-container">
-        <div id="background"></div>
+        <div id="background-clip">
+          <div id="background"></div>
+        </div>
         <div id="date-accessed" class="card-title">
           [[cardTitle_(numberOfItems, item.dateRelativeDay, searchTerm)]]
         </div>
         <div id="item-container">
           <paper-checkbox id="checkbox" on-mousedown="onCheckboxMousedown_"
               on-click="onCheckboxSelected_" checked="{{selected}}"
               disabled="[[selectionNotAllowed_()]]"
               aria-label$="[[getEntrySummary_(item)]]">
           </paper-checkbox>
           <span id="time-accessed">[[item.readableTimestamp]]</span>
@@ skipping 22 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>