[MD Bookmarks] Persist collapsed folders between page loads.

chrome/test/data/webui/md_bookmarks/test_store.js

Index: chrome/test/data/webui/md_bookmarks/test_store.js
diff --git a/chrome/test/data/webui/md_bookmarks/test_store.js b/chrome/test/data/webui/md_bookmarks/test_store.js
index 77f5c0fee158a3f7cc0d891cd294c15a5e833e7d..e99f3896c4eab0b4bc108c16c72d7f6a1d9015db 100644
--- a/chrome/test/data/webui/md_bookmarks/test_store.js
+++ b/chrome/test/data/webui/md_bookmarks/test_store.js
@@ -7,6 +7,7 @@ cr.define('bookmarks', function() {
     this.data = Object.assign(bookmarks.util.createEmptyState(), data);
     this.lastAction_ = null;
     this.observers_ = [];
+    this.acceptInit_ = false;
   };
 
   TestStore.prototype = {
@@ -14,7 +15,10 @@ cr.define('bookmarks', function() {
       this.observers_.push(client);
     },
 
-    init: function() {},
+    init: function(state) {
+      if (this.acceptInit_)
+        this.data = state;
+    },
 
     removeObserver: function(client) {},
 
@@ -37,6 +41,11 @@ cr.define('bookmarks', function() {
       // StoreClient is updated.
       this.observers_.forEach((client) => client.onStateChanged(this.data));
     },
+
+    // Call in order to accept data from an init call to the TestStore.
+    acceptInit: function() {
+      this.acceptInit_ = true;
+    },
   };
 
   return {