MD History: Add basic material design history cards and history items

chrome/browser/resources/md_history/history_card.js

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+Polymer({
+  is: 'history-card',
+
+  properties: {
+    // The date of these history items.
+    historyDate: {
+      type: String,
+      value: ''
+    },

[Dan Beam, 2016/01/27 03:07:30]

nit: \n between props

[yingran, 2016/01/27 05:05:03]

Done.

[Dan Beam, 2016/01/27 18:20:07]

(not yet done)

[yingran, 2016/01/28 00:51:00]

Done.

+    // The list of history results that were accessed for a particular day in
+    // reverse chronological order.
+    historyItems: {
+      type: Array,
+      value: function() { return []; }
+    }
+  },
+
+  /**
+   * Check whether the time difference between the given historyItem and the
+   * next one is large enough for a spacer to be required.
+   * @param {number} index The index number of the first item being compared.
+   * @return {boolean} Whether or not time gap separator is required.
+   * @private
+   */
+  needsTimeGap_: function(index) {
+    var items = this.historyItems;
+    return index + 1 < items.length &&
+        items[index].time - items[index + 1].time > BROWSING_GAP_TIME;
+  }
+});