DevTools: expose layer compositing reasons and paint count via layer tree agent

LayoutTests/inspector/layers-test.js

 function initialize_LayerTreeTests()
 {
     // FIXME: remove once out of experimental.
     WebInspector.inspectorView.addPanel(new WebInspector.LayersPanelDescriptor());
     InspectorTest._layerTreeModel = WebInspector.panel("layers")._model;
 
     InspectorTest.labelForLayer = function(layer)
     {
         var node = WebInspector.domAgent.nodeForId(layer.nodeIdForSelfOrAncestor());
         var label = node.appropriateSelectorFor(false);
@@ skipping 40 matching lines @@
         {
             if (!layer.nodeId())
                 return false;
             var node = WebInspector.domAgent.nodeForId(layer.nodeId());
             if (!node || node.getAttribute("id") !== nodeIdAttribute)
                 return false;
             result = layer;
             return true;
         }
         InspectorTest._layerTreeModel.forEachLayer(testLayer);
+        if (!result)
+            InspectorTest.addResult("ERROR: No layer for " + nodeIdAttribute);
         return result;
     }
 }