bluetooth: Add characteristic list to DeviceDetailsPage in internals page.

chrome/browser/resources/bluetooth_internals/expandable_list.js

Index: chrome/browser/resources/bluetooth_internals/expandable_list.js
diff --git a/chrome/browser/resources/bluetooth_internals/expandable_list.js b/chrome/browser/resources/bluetooth_internals/expandable_list.js
index b367e9f873eb301b7717dd4ea5b403df934dbd83..c9cff05e290da15e5b67eec2f1158d664079d43e 100644
--- a/chrome/browser/resources/bluetooth_internals/expandable_list.js
+++ b/chrome/browser/resources/bluetooth_internals/expandable_list.js
@@ -40,11 +40,17 @@ cr.define('expandable_list', function() {
     },
 
     /**
+     * Called when the list item is expanded or collapsed.
+     * @param {boolean} expanded
+     */
+    onExpand: function(expanded) {},
+
+    /**
      * Toggles the expanded class on the item.
      * @private
      */
     toggleExpand_: function() {
-      this.classList.toggle('expanded');
+      this.onExpand(this.classList.toggle('expanded'));
     },
   };
 
@@ -112,6 +118,14 @@ cr.define('expandable_list', function() {
     },
 
     /**
+     * Gets the loading state of the list. Returns true if the list is loading.
+     * @return {boolean}
+     */
+    isLoading: function() {
+      return !this.spinner_.hidden;
+    },
+
+    /**
      * Updates the display state of the empty message. If there are no items in
      * the data model, the empty message is displayed.
      */