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

third_party/WebKit/Source/devtools/front_end/sdk/LayerTreeBase.js

Index: third_party/WebKit/Source/devtools/front_end/sdk/LayerTreeBase.js
diff --git a/third_party/WebKit/Source/devtools/front_end/sdk/LayerTreeBase.js b/third_party/WebKit/Source/devtools/front_end/sdk/LayerTreeBase.js
index 2de99e2151685a93504ad43f3454ea7a90ed4178..6dc5b8e552344653f70cc6897eea255fd0b4bf59 100644
--- a/third_party/WebKit/Source/devtools/front_end/sdk/LayerTreeBase.js
+++ b/third_party/WebKit/Source/devtools/front_end/sdk/LayerTreeBase.js
@@ -150,7 +150,7 @@ SDK.LayerTreeBase = class {
     this._layersById = {};
     this._root = null;
     this._contentRoot = null;
-    /** @type Map<number, ?SDK.DOMNode> */
+    /** @type {!Map<number, ?SDK.DOMNode>} */
     this._backendNodeIdToNode = new Map();
   }
 
@@ -217,7 +217,7 @@ SDK.LayerTreeBase = class {
    * @param {!Set<number>} requestedNodeIds
    * @param {function()} callback
    */
-  _resolveBackendNodeIds(requestedNodeIds, callback) {
+  resolveBackendNodeIds(requestedNodeIds, callback) {
     if (!requestedNodeIds.size || !this._domModel) {
       callback();
       return;
@@ -239,6 +239,13 @@ SDK.LayerTreeBase = class {
   }
 
   /**
+   * @return {!Map<number, ?SDK.DOMNode>}
+   */
+  backendNodeIdToNode() {
+    return this._backendNodeIdToNode;
+  }
+
+  /**
    * @param {!{width: number, height: number}} viewportSize
    */
   setViewportSize(viewportSize) {