[MD Bookmarks] Add draggable sidebar.

chrome/browser/resources/md_bookmarks/toolbar.js

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 Polymer({
   is: 'bookmarks-toolbar',
 
   properties: {
     searchTerm: {
       type: String,
       observer: 'onSearchTermChanged_',
     },
+
+    sidebarWidth: {
+      type: String,
+      observer: 'onSidebarWidthChanged_',
+    },
   },
 
   /** @return {CrToolbarSearchFieldElement} */
   get searchField() {
     return /** @type {CrToolbarElement} */ (this.$$('cr-toolbar'))
         .getSearchField();
   },
 
   /**
    * @param {Event} e
@@ skipping 39 matching lines @@
 
   /**
    * @param {Event} e
    * @private
    */
   onSearchChanged_: function(e) {
     var searchTerm = /** @type {string} */ (e.detail);
     this.fire('search-term-changed', searchTerm);
   },
 
+  onSidebarWidthChanged_: function() {
+    this.style.setProperty('--sidebar-width', this.sidebarWidth);
+  },
+
   /** @private */
   onSearchTermChanged_: function() {
     this.searchField.setValue(this.searchTerm || '');
   },
 });