[MD History] Factor out a common HistoryListBehavior.

chrome/browser/resources/md_history/history_item.js

Index: chrome/browser/resources/md_history/history_item.js
diff --git a/chrome/browser/resources/md_history/history_item.js b/chrome/browser/resources/md_history/history_item.js
index 5a3fa265364c27cd8df7618ddb807c0c770451e1..7a1133f5d72f028f0a2c6e672faeb24c4d5d8b12 100644
--- a/chrome/browser/resources/md_history/history_item.js
+++ b/chrome/browser/resources/md_history/history_item.js
@@ -31,7 +31,10 @@ cr.define('md_history', function() {
 
       hasTimeGap: {type: Boolean},
 
-      numberOfItems: {type: Number}
+      numberOfItems: {type: Number},
+
+      // The path of this history item inside its parent.
+      path: String,
     },
 
     /**
@@ -40,7 +43,9 @@ cr.define('md_history', function() {
      * @private
      */
     onCheckboxSelected_: function() {
+      // TODO(calamity): Fire this event whenever |selected| changes.
       this.fire('history-checkbox-select', {
+        element: this,
         countAddition: this.$.checkbox.checked ? 1 : -1
       });
     },