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 2f4d146c96f58e060dcd895fef5d4020709af77c..ec299e68cb82490d9693f53ff310a896da3b8989 100644
--- a/Source/core/inspector/InspectorLayerTreeAgent.cpp
+++ b/Source/core/inspector/InspectorLayerTreeAgent.cpp
@@ -197,6 +197,10 @@ void InspectorLayerTreeAgent::getLayers(ErrorString* errorString, const int* nod
         return;
     }
     RenderLayer* enclosingLayer = renderer->enclosingLayer();
+    if (!enclosingLayer->enclosingCompositingLayer() || !enclosingLayer->enclosingCompositingLayer()->compositedLayerMapping()) {
+        *errorString = "No CompositedLayerMapping for the provided node";

[caseq, 2013/10/17 12:37:38]

When are we supposed to hit this? enclosingCompositingLayer() returns only
layers for which isComposited() is true, and the latter just checks for the
presence for compositedLayerMapping, so we should never hit the second clause.
Hitting the first one means we traversed up till the root layer without
encountering a composited layer.
Is this for cases where we've done the style recalc and layout but not yet
updated the layer tree? I think we either need to force layer tree update in
this case, or rather make this function async and actually invoke the callback
only after we've rebuilt the layer tree. The former is cheaper to implement
though less efficient (which probably does not matter, as we throttle calls to
this function on the front-end). The latter is a bit more tricky, but I can help
with it if the "cheap" approach fails (it looks like
RenderLayerCompositor::updateCompositingLayers() bails out in case layout is not
yet done, so if we're unlucky to invoke getLayers() at the moment when layouit
is invalid, a call to updateCompositingLayers() will have no effect).

[shawnsingh, 2013/10/17 19:00:05]

Yeah - as you suspected, this issue occurs when compositing is not updated yet. 
You're also right that we may never be hitting the second condition on the if
statement - I need to double-check this.

When I tried forcing a compositing update here, it was crashing deep in
updateCompositingLayers().  It did not early exit because layout was incomplete.
 The scenario is that layout is done, but compositing is not yet updated, so no
compositedLayerMappings are yet allocated, not even for the root.  I will try to
understand more detail about why forcing an update here was not working.

+        return;
+    }
     GraphicsLayer* enclosingGraphicsLayer = enclosingLayer->enclosingCompositingLayer()->compositedLayerMapping()->childForSuperlayers();
     buildLayerIdToNodeIdMap(errorString, enclosingLayer, layerIdToNodeIdMap);
     gatherGraphicsLayers(enclosingGraphicsLayer, layerIdToNodeIdMap, layers);