Replace LayerImpl::sorting_context_id with transform reference.

third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.cpp

Index: third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.cpp
diff --git a/third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.cpp b/third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.cpp
index 036586341f78c05f5737cfd7675d1a319dd5883c..3a37e21d2770c2846db410306bbb49c434652ff8 100644
--- a/third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.cpp
+++ b/third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.cpp
@@ -1174,8 +1174,6 @@ void PaintArtifactCompositor::update(
     layer->SetEffectTreeIndex(effectId);
     layer->SetScrollTreeIndex(scrollId);
 
-    // TODO(jbroman): This probably shouldn't be necessary, but it is still
-    // queried by RenderSurfaceImpl.

[ajuma, 2017/01/05 00:22:20]

In theory, I think looking up the sorting context id from a LayerImpl's
transform node instead of having this value on the LayerImpl itself should work,
since cc's property tree builder does add a new transform node whenever the
sorting context id changes, see
https://cs.chromium.org/chromium/src/cc/trees/property_tree_builder.cc?rcl=0&....

[chrishtr, 2017/01/05 01:04:03]

Correct, this is how it should work. Ali, should that work already then?

[ajuma, 2017/01/05 14:13:21]

Yes, changing all the places in cc that use LayerImpl"s sorting_context_id to
using its transform node's sorting context id should work already (there might
be unit tests where we aren't building/rebuilding property trees, but those
should be fixable just by making sure property trees get built).

     layer->Set3dSortingContextId(layerTree->property_trees()
                                      ->transform_tree.Node(transformId)
                                      ->sorting_context_id);