[MD History] Implement grouped history UI.

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-flex-layout/iron-flex-layout.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.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/constants.html">
 <link rel="import" href="chrome://history/shared_style.html">
 
 <dom-module id="history-item">
   <template>
     <style include="shared-style">
       :host {
         @apply(--layout-center);
         @apply(--layout-vertical);
-        padding: 0 var(--card-padding-side);
+        padding: 0 var(--history-item-padding-side, 0);
       }
 
       #main-container {
         background: #fff;
         border-color: var(--card-border-color);
-        border-style: solid;
+        border-style: var(--border-style, solid);
         border-width: 0 1px;
         max-width: var(--card-max-width);
         min-width: var(--card-min-width);
         width: 100%;
       }
 
       :host([is-card-start]) #main-container {
         border-top-width: 1px;
       }
 
       :host([is-card-end]) #main-container {
         border-bottom-width: 2px;
-        margin-bottom: 20px;
+        margin-bottom: var(--card-padding-between);
       }
 
       #date-accessed {
         display: none;
       }
 
       :host([is-card-start]) #date-accessed {
         display: flex;
       }
 
       #item-container {
         @apply(--layout-center);
         @apply(--layout-horizontal);
         min-height: 40px;
       }
 
       :host([is-card-start]) #item-container {
-        padding-top: 8px;
+        padding-top: var(--card-first-last-item-padding);
       }
 
       :host([is-card-end]) #item-container {
-        padding-bottom: 8px;
+        padding-bottom: var(--card-first-last-item-padding);
       }
 
       #title-and-domain {
         @apply(--layout-center);
         @apply(--layout-flex);
         @apply(--layout-horizontal);
         min-height: 40px;
         overflow: hidden;
       }
 
@@ skipping 66 matching lines @@
             on-tap="onMenuButtonTap_">
         </paper-icon-button>
       </div>
       <template is="dom-if" if="[[hasTimeGap]]">
         <div id="time-gap-separator"></div>
       </template>
     </div>
   </template>
   <script src="chrome://history/history_item.js"></script>
 </dom-module>