MD History: Update timestamp hover text when item changes

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/cr/ui/focus_row.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-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">
@@ skipping 199 matching lines @@
       <div id="item-container"
           on-mousedown="onItemMousedown_"
           on-click="onItemClick_">
         <button is="paper-icon-button-light" id="checkbox"
             hidden="[[selectionNotAllowed_]]"
             role="checkbox"
             aria-checked$="[[getAriaChecked_(selected)]]"
             aria-label$="[[getEntrySummary_(item)]]">
           <div id="checkmark"></div>
         </button>
-        <span id="time-accessed" on-mouseover="addTimeTitle_">
+        <span id="time-accessed">
           [[item.readableTimestamp]]
         </span>
         <div class="website-icon" id="icon"></div>
         <div id="title-and-domain">
           <a href="[[item.url]]" id="title" class="website-title"
               title="[[item.title]]" on-click="onLinkClick_"
               on-contextmenu="onLinkRightClick_">
             <history-searched-label title="[[item.title]]"
                 search-term="[[searchTerm]]"></history-searched-label>
           </a>
@@ skipping 13 matching lines @@
           <div></div>
           <div></div>
           <div></div>
         </button>
       </div>
       <div id="time-gap-separator" hidden="[[!hasTimeGap]]"></div>
     </div>
   </template>
   <script src="chrome://history/history_item.js"></script>
 </dom-module>