Defer the real work in updateCompositingLayers until it's really needed.

Source/core/inspector/InspectorLayerTreeAgent.cpp

Index: Source/core/inspector/InspectorLayerTreeAgent.cpp
diff --git a/Source/core/inspector/InspectorLayerTreeAgent.cpp b/Source/core/inspector/InspectorLayerTreeAgent.cpp
index b91d04a1051f2e6bc6a8837bea0ff71f4421c2e5..2498ae681bd4d2835f7335202b8eebd2e6ba2193 100644
--- a/Source/core/inspector/InspectorLayerTreeAgent.cpp
+++ b/Source/core/inspector/InspectorLayerTreeAgent.cpp
@@ -181,6 +181,8 @@ void InspectorLayerTreeAgent::getLayers(ErrorString* errorString, const int* nod
         *errorString = "Not in the compositing mode";
         return;
     }
+    // FIXME: when inspector layer access is made asynchronous, this will probably need to be changed.
+    compositor->updateCompositingLayers(CompositingUpdateFinishAllDeferredWork);
     if (!nodeId) {
         buildLayerIdToNodeIdMap(errorString, compositor->rootRenderLayer(), layerIdToNodeIdMap);
         gatherGraphicsLayers(compositor->rootGraphicsLayer(), layerIdToNodeIdMap, layers);
@@ -197,6 +199,10 @@ void InspectorLayerTreeAgent::getLayers(ErrorString* errorString, const int* nod
         return;
     }
     RenderLayer* enclosingLayer = renderer->enclosingLayer();
+    if (!enclosingLayer->enclosingCompositingLayer() || !enclosingLayer->enclosingCompositingLayer()->compositedLayerMapping()) {

[caseq, 2013/10/18 05:39:56]

I think this is never supposed to be true now that the layer tree should be up
to date at this point. I suggest we either drop this or perhaps drop the second
clause and change the first one to an assert.

+        *errorString = "No CompositedLayerMapping for the provided node";
+        return;
+    }
     GraphicsLayer* enclosingGraphicsLayer = enclosingLayer->enclosingCompositingLayer()->compositedLayerMapping()->childForSuperlayers();
     buildLayerIdToNodeIdMap(errorString, enclosingLayer, layerIdToNodeIdMap);
     gatherGraphicsLayers(enclosingGraphicsLayer, layerIdToNodeIdMap, layers);