[MD History] Add keyboard navigation to the main history list.

chrome/browser/resources/md_history/history_list.html

 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-list/iron-list.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-scroll-threshold/iron-scroll-threshold.html">
 <link rel="import" href="chrome://history/constants.html">
 <link rel="import" href="chrome://history/history_item.html">
 <link rel="import" href="chrome://history/history_list_behavior.html">
 <link rel="import" href="chrome://history/shared_style.html">
 
 <dom-module id="history-list">
   <template>
     <style include="shared-style">
       :host {
         display: block;
         overflow: auto;
       }
 
       history-item {
         --history-item-padding-side: var(--card-padding-side);
       }
     </style>
     <div id="no-results" class="centered-message"
         hidden$="[[hasResults(historyData_.length)]]">
       {{noResultsMessage(searchedTerm, querying)}}
     </div>
     <iron-list items="{{historyData_}}" as="item" id="infinite-list"
         hidden$="[[!hasResults(historyData_.length)]]">
       <template>
-        <history-item item="[[item]]"
+        <history-item tabindex="-1"
+            item="[[item]]"
             selected="{{item.selected}}"
             is-first-item="[[isFirstItem_(index)]]"
             is-card-start="[[isCardStart_(item, index, historyData_.length)]]"
             is-card-end="[[isCardEnd_(item, index, historyData_.length)]]"
             has-time-gap="[[needsTimeGap_(item, index, historyData_.length)]]"
             search-term="[[searchedTerm]]"
             number-of-items="[[historyData_.length]]"
             path="[[pathForItem_(index)]]"
-            index="[[index]]">
+            index="[[index]]"
+            tab-index="[[tabIndex]]"

[Dan Beam, 2016/09/15 16:51:10]

can we just name this something else?  iron-list-tab-index?

this might invoke HTMLElement#tabIndex, which is a DOM 0 accessor
semi-equivalent to setAttribute('tabindex', ...)

+            last-focused="{{lastFocused_}}">
         </history-item>
       </template>
     </iron-list>
     <iron-scroll-threshold id="scroll-threshold" scroll-target="infinite-list"
         lower-threshold="500" on-lower-threshold="loadMoreData_">
     </iron-scroll-threshold>
   </template>
   <script src="chrome://history/history_list.js"></script>
 </dom-module>