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

Source/core/inspector/InspectorLayerTreeAgent.cpp

 /*
  * Copyright (C) 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
@@ skipping 163 matching lines @@
 void InspectorLayerTreeAgent::getLayers(ErrorString* errorString, const int* nodeId, RefPtr<TypeBuilder::Array<TypeBuilder::LayerTree::Layer> >& layers)
 {
     LayerIdToNodeIdMap layerIdToNodeIdMap;
     layers = TypeBuilder::Array<TypeBuilder::LayerTree::Layer>::create();
 
     RenderLayerCompositor* compositor = renderLayerCompositor();
     if (!compositor || !compositor->inCompositingMode()) {
         *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);
         return;
     }
     Node* node = m_instrumentingAgents->inspectorDOMAgent()->nodeForId(*nodeId);
     if (!node) {
         *errorString = "Provided node id doesn't match any known node";
         return;
     }
     RenderObject* renderer = node->renderer();
     if (!renderer) {
         *errorString = "Node for provided node id doesn't have a renderer";
         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);
 }
 
 void InspectorLayerTreeAgent::buildLayerIdToNodeIdMap(ErrorString* errorString, RenderLayer* root, LayerIdToNodeIdMap& layerIdToNodeIdMap)
 {
     if (root->compositedLayerMapping()) {
         if (Node* node = root->renderer()->generatingNode()) {
             GraphicsLayer* graphicsLayer = root->compositedLayerMapping()->childForSuperlayers();
@@ skipping 114 matching lines @@
             continue;
         reasonStrings->addItem(compositingReasonNames[i].protocolName);
 #ifndef _NDEBUG
         reasonsBitmask &= ~compositingReasonNames[i].mask;
 #endif
     }
     ASSERT(!reasonsBitmask);
 }
 
 } // namespace WebCore