Added TodoMVC startup benchmarks.

samples/third_party/todomvc_performance/js_todomvc/elements/td-model.html

+<link rel="import" href="../components/polymer/polymer.html">
+
+<polymer-element name="td-model" attributes="filter items storageId">
+        <script>
+                Polymer('td-model', {
+                        filtered: null,
+                        completedCount: 0,
+                        activeCount: 0,
+                        allCompleted: false,
+                        ready: function() {
+                                this.asyncMethod(function() {
+                                        this.items = this.items || [];
+                                });
+                        },
+                        filterChanged: function() {
+                                this.filterItems();
+                        },
+                        itemsChanged: function() {
+                                this.completedCount =
+                                        this.items.filter(this.filters.completed).length;
+                                this.activeCount = this.items.length - this.completedCount;
+                                this.allCompleted = this.completedCount && !this.activeCount;
+                                this.filterItems();
+                                if (this.storage) {
+                                        this.storage.value = this.items;
+                                        this.storage.save();
+                                }
+                        },
+                        storageIdChanged: function() {
+                                this.storage = document.querySelector('#' + this.storageId);
+                                this.storage && (this.items = this.storage.value);
+                        },
+                        filterItems: function() {
+                                var fn = this.filters[this.filter];
+                                this.filtered = fn ? this.items.filter(fn) : this.items;
+                        },
+                        newItem: function(title) {
+                                title = String(title).trim();
+                                if (title) {
+                                        var item = {
+                                                title: title,
+                                                completed: false
+                                        };
+                                        this.items.push(item);
+                                        this.itemsChanged();
+                                }
+                        },
+                        destroyItem: function(item) {
+                                var i = this.items.indexOf(item);
+                                if (i >= 0) {
+                                        this.items.splice(i, 1);
+                                }
+                                this.itemsChanged();
+                        },
+                        clearItems: function(){
+                                this.items = this.items.filter(this.filters.active);
+                        },
+                        setItemsCompleted: function(completed) {
+                                this.items.forEach(function(item) {
+                                        item.completed = completed;
+                                });
+                                this.itemsChanged();
+                        },
+                        filters: {
+                                active: function(item) {
+                                        return !item.completed;
+                                },
+                                completed: function(item) {
+                                        return item.completed;
+                                }
+                        }
+         });
+        </script>
+</polymer-element>