Don't coerce pointers to compositor layer mappings to booleans.

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 1d56a28c3ec98b72066f643086df81a947df14ac..3cbe21efb3222fab65b69e2e5239e5314eac8a61 100644
--- a/Source/core/inspector/InspectorLayerTreeAgent.cpp
+++ b/Source/core/inspector/InspectorLayerTreeAgent.cpp
@@ -152,11 +152,13 @@ void InspectorLayerTreeAgent::layerTreeDidChange()
 void InspectorLayerTreeAgent::didPaint(RenderObject* renderer, GraphicsContext*, const LayoutRect& rect)
 {
     RenderLayer* renderLayer = toRenderLayerModelObject(renderer)->layer();
-    CompositedLayerMapping* compositedLayerMapping = renderLayer->compositedLayerMapping();
     // Should only happen for FrameView paints when compositing is off. Consider different instrumentation method for that.
-    if (!compositedLayerMapping)
+    // Note, that this only gets called with the FrameView's RenderView which, if it is composited, only

[caseq, 2013/11/13 15:45:20]

This statement is not correct, there are two call sites for
InspectorInstrumentation::didPaint() -- one in RenderView, another in
CompositedLayerMapping. The former is on the non-composited code path only (so
all we have to do here is bail out) and the latter is naturally on composited
path. The ASSERT() below looks reasonable, but I'd move it after if
(renderLayer->compositingState() != PaintsIntoOwnBacking) and remove the first
clause.

+    // paint into its own backing.

[shawnsingh, 2013/11/12 09:55:50]

s/paint/paints/

+    ASSERT(renderLayer->compositingState() == PaintsIntoOwnBacking || renderLayer->compositingState() == NotComposited);
+    if (renderLayer->compositingState() != PaintsIntoOwnBacking)
         return;
-    GraphicsLayer* graphicsLayer = compositedLayerMapping->mainGraphicsLayer();
+    GraphicsLayer* graphicsLayer = renderLayer->compositedLayerMapping()->mainGraphicsLayer();
     RefPtr<TypeBuilder::DOM::Rect> domRect = TypeBuilder::DOM::Rect::create()
         .setX(rect.x())
         .setY(rect.y())
@@ -179,7 +181,11 @@ PassRefPtr<TypeBuilder::Array<TypeBuilder::LayerTree::Layer> > InspectorLayerTre
 
 void InspectorLayerTreeAgent::buildLayerIdToNodeIdMap(RenderLayer* root, LayerIdToNodeIdMap& layerIdToNodeIdMap)
 {
-    if (root->compositedLayerMapping()) {
+    // The layer-id to node-id map is used later in buildObjectForLayer to map each graphics
+    // layer back to its corresponding Node. We only want to map a graphics layer to map back
+    // to the Node corresponding to its owning RenderLayer, not some other RenderLayer that
+    // paints into that GraphicsLayer.
+    if (root->compositingState() == PaintsIntoOwnBacking) {

[shawnsingh, 2013/11/12 09:55:50]

Isn't this a scenario where we really only care whether the
compositedLayerMapping exists or not, so that the layer-to-node map represents
the same information as RenderLayer-owns-CLM relationships? If you disagree, can
you please explain more clearly why we cannot just use
hasCompositedLayerMapping()?

[caseq, 2013/11/13 15:45:20]

+1 to what Shawn says.
Since the mere presence of CLM means we do have at least a main graphics layer,
event if this render layer is painted elsewhere, we want that layer to be
associated with the actual owner node that caused it to appear.

[Ian Vollick, 2013/11/14 04:12:47]

Agreed.

         if (Node* node = root->renderer()->generatingNode()) {
             GraphicsLayer* graphicsLayer = root->compositedLayerMapping()->childForSuperlayers();
             layerIdToNodeIdMap.set(graphicsLayer->platformLayer()->id(), idForNode(node));