[MD Bookmarks] Make folder nodes extend their whole width.

chrome/browser/resources/md_bookmarks/folder_node.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-folder-node',
 
   behaviors: [
     bookmarks.StoreClient,
   ],
 
   properties: {
     itemId: {
       type: String,
       observer: 'updateFromStore',
     },
 
+    depth: {
+      type: Number,
+      observer: 'depthChanged_',
+    },
+
     /** @type {BookmarkNode} */
     item_: Object,
 
     /** @private */
     isClosed_: Boolean,
 
     /** @private */
     selectedFolder_: String,
 
     /** @private */
@@ skipping 43 matching lines @@
   /**
    * Occurs when the drop down arrow is tapped.
    * @private
    */
   toggleFolder_: function() {
     this.dispatch(
         bookmarks.actions.changeFolderOpen(this.item_.id, this.isClosed_));
   },
 
   /**
+   * @private
    * @param {string} itemId
    * @param {string} selectedFolder
    * @return {boolean}
-   * @private
    */
   computeIsSelected_: function(itemId, selectedFolder) {
     return itemId == selectedFolder;
   },
 
   /**
    * @private
    * @return {boolean}
    */
   hasChildFolder_: function() {
     for (var i = 0; i < this.item_.children.length; i++) {
       if (this.isFolder_(this.item_.children[i]))
         return true;
     }
     return false;
   },
 
+  /** @private */
+  depthChanged_: function() {
+    this.style.setProperty('--node-depth', this.depth.toString());
+  },
+
+  /**
+   * @private
+   * @return {number}
+   */
+  getChildDepth_: function() {
+    return this.depth + 1;
+  },
+
   /**
    * @param {string} itemId
    * @private
    * @return {boolean}
    */
   isFolder_: function(itemId) {
     return !this.getState().nodes[itemId].url;
   }
 });