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

LayoutTests/inspector/layers-test.js

Index: LayoutTests/inspector/layers-test.js
diff --git a/LayoutTests/inspector/layers-test.js b/LayoutTests/inspector/layers-test.js
index f038bfbce25bccbc2c1af412948f61f5ce3e84e8..070fe01ec0b4747d1959b2212580b529145212f1 100644
--- a/LayoutTests/inspector/layers-test.js
+++ b/LayoutTests/inspector/layers-test.js
@@ -58,6 +58,8 @@ function initialize_LayerTreeTests()
             return true;
         }
         InspectorTest._layerTreeModel.forEachLayer(testLayer);
+        if (!result)
+            InspectorTest.addResult("ERROR: No layer for " + nodeIdAttribute);
         return result;
     }
 }