[MD Bookmarks] Change selection items from Map to Set.

chrome/browser/resources/md_bookmarks/util.js

Index: chrome/browser/resources/md_bookmarks/util.js
diff --git a/chrome/browser/resources/md_bookmarks/util.js b/chrome/browser/resources/md_bookmarks/util.js
index 1227a61f01ca3d59a06d5884ac2ffa083075ad11..26d209f0c20cb972cf8ca66e5dce4852ab789fe8 100644
--- a/chrome/browser/resources/md_bookmarks/util.js
+++ b/chrome/browser/resources/md_bookmarks/util.js
@@ -54,14 +54,14 @@ cr.define('bookmarks.util', function() {
     return {
       nodes: {},
       selectedFolder: '0',
-      closedFolders: {},
+      closedFolders: new Set(),
       search: {
         term: '',
         inProgress: false,
         results: [],
       },
       selection: {
-        items: {},
+        items: new Set(),
         anchor: null,
       },
     };