[MD Bookmarks] Persist collapsed folders between page loads.

chrome/browser/resources/md_bookmarks/app.js

Index: chrome/browser/resources/md_bookmarks/app.js
diff --git a/chrome/browser/resources/md_bookmarks/app.js b/chrome/browser/resources/md_bookmarks/app.js
index 1c0983e0992604ffedfad62ad5fbc3fbd725b948..03cddd400aa140fd520c300d186f291ce55fdb96 100644
--- a/chrome/browser/resources/md_bookmarks/app.js
+++ b/chrome/browser/resources/md_bookmarks/app.js
@@ -36,8 +36,13 @@ Polymer({
       var nodeList = bookmarks.util.normalizeNodes(results[0]);
       var initialState = bookmarks.util.createEmptyState();
       initialState.nodes = nodeList;
-      initialState.selectedFolder =
-          nodeList[bookmarks.util.ROOT_NODE_ID].children[0];
+      initialState.selectedFolder = nodeList[ROOT_NODE_ID].children[0];
+      var closedFoldersString =
+          window.localStorage[LOCAL_STORAGE_CLOSED_FOLDERS_KEY];
+      initialState.closedFolders = closedFoldersString ?
+          /** @type {!Object<string,boolean>} */ (
+              JSON.parse(closedFoldersString)) :
+          {};
 
       bookmarks.Store.getInstance().init(initialState);
       bookmarks.ApiListener.init();