DevTools: avoid using stale front-end node ids.

LayoutTests/http/tests/inspector/layers-test.js

 function initialize_LayerTreeTests()
 {
     // FIXME: remove once out of experimental.
     WebInspector.moduleManager.registerModule("layers");
     var extensions = WebInspector.moduleManager.extensions(WebInspector.Panel).forEach(function(extension) {
         if (extension.module().name() === "layers")
             WebInspector.inspectorView.addPanel(new WebInspector.ModuleManagerExtensionPanelDescriptor(extension));
     });
     InspectorTest.layerTreeModel = WebInspector.inspectorView.panel("layers")._model;
 
     InspectorTest.labelForLayer = function(layer)
     {
         var node = WebInspector.domAgent.nodeForId(layer.nodeIdForSelfOrAncestor());
-        var label = WebInspector.DOMPresentationUtils.fullQualifiedSelector(node, false);
+        var label = node ? WebInspector.DOMPresentationUtils.fullQualifiedSelector(node, false) : "<invalid node id>";
         var height = layer.height();
         var width = layer.width();
         if (height <= 200 && width <= 200)
             label += " " + height + "x" + width;
         if (typeof layer.__extraData !== "undefined")
             label += " (" + layer.__extraData + ")";
         return label;
     }
 
     InspectorTest.dumpLayerTree = function(prefix, root)
@@ skipping 70 matching lines @@
     {
         InspectorTest.layerTreeModel.addEventListener(WebInspector.LayerTreeModel.Events.LayerTreeChanged, onLayerTreeChanged);
         InspectorTest.layerTreeModel.enable();
         function onLayerTreeChanged()
         {
             InspectorTest.layerTreeModel.removeEventListener(WebInspector.LayerTreeModel.Events.LayerTreeChanged, onLayerTreeChanged);
             callback();
         }
     }
 }