DevTools: fix private field usage violation discovered after the compiler roll.

third_party/WebKit/Source/devtools/front_end/sdk/LayerTreeBase.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.
 /** @typedef {!{
         rect: !Protocol.DOM.Rect,
         snapshot: !SDK.PaintProfilerSnapshot
     }}
 */
 SDK.SnapshotWithRect;
 
@@ skipping 132 matching lines @@
 SDK.LayerTreeBase = class {
   /**
    * @param {?SDK.Target} target
    */
   constructor(target) {
     this._target = target;
     this._domModel = target ? SDK.DOMModel.fromTarget(target) : null;
     this._layersById = {};
     this._root = null;
     this._contentRoot = null;
-    /** @type Map<number, ?SDK.DOMNode> */
+    /** @type {!Map<number, ?SDK.DOMNode>} */
     this._backendNodeIdToNode = new Map();
   }
 
   /**
    * @return {?SDK.Target}
    */
   target() {
     return this._target;
   }
 
@@ skipping 46 matching lines @@
    * @return {?SDK.Layer}
    */
   layerById(id) {
     return this._layersById[id] || null;
   }
 
   /**
    * @param {!Set<number>} requestedNodeIds
    * @param {function()} callback
    */
-  _resolveBackendNodeIds(requestedNodeIds, callback) {
+  resolveBackendNodeIds(requestedNodeIds, callback) {
     if (!requestedNodeIds.size || !this._domModel) {
       callback();
       return;
     }
     if (this._domModel)
       this._domModel.pushNodesByBackendIdsToFrontend(requestedNodeIds, populateBackendNodeMap.bind(this));
 
     /**
      * @this {SDK.LayerTreeBase}
      * @param {?Map<number, ?SDK.DOMNode>} nodesMap
      */
     function populateBackendNodeMap(nodesMap) {
       if (nodesMap) {
         for (var nodeId of nodesMap.keysArray())
           this._backendNodeIdToNode.set(nodeId, nodesMap.get(nodeId) || null);
       }
       callback();
     }
   }
 
   /**
+   * @return {!Map<number, ?SDK.DOMNode>}
+   */
+  backendNodeIdToNode() {
+    return this._backendNodeIdToNode;
+  }
+
+  /**
    * @param {!{width: number, height: number}} viewportSize
    */
   setViewportSize(viewportSize) {
     this._viewportSize = viewportSize;
   }
 
   /**
    * @return {!{width: number, height: number}|undefined}
    */
   viewportSize() {
     return this._viewportSize;
   }
 
   /**
    * @param {number} id
    * @return {?SDK.DOMNode}
    */
   _nodeForId(id) {
     return this._domModel ? this._domModel.nodeForId(id) : null;
   }
 };