Replace LayerImpl::sorting_context_id with transform reference.

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

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "platform/graphics/compositing/PaintArtifactCompositor.h"
 
 #include "cc/layers/content_layer_client.h"
 #include "cc/layers/layer.h"
 #include "cc/layers/picture_layer.h"
 #include "cc/playback/compositing_display_item.h"
@@ skipping 1156 matching lines @@
 
     layer->set_offset_to_transform_parent(layerOffset);
 
     m_rootLayer->AddChild(layer);
     layer->set_property_tree_sequence_number(kPropertyTreeSequenceNumber);
     layer->SetTransformTreeIndex(transformId);
     layer->SetClipTreeIndex(clipId);
     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);
 
     layer->SetShouldCheckBackfaceVisibility(pendingLayer.backfaceHidden);
 
     if (m_extraDataForTestingEnabled)
       m_extraDataForTesting->contentLayers.append(layer);
   }
   m_contentLayerClients.clear();
   m_contentLayerClients.swap(newContentLayerClients);
 
   // Mark the property trees as having been rebuilt.
   layerTree->property_trees()->sequence_number = kPropertyTreeSequenceNumber;
   layerTree->property_trees()->needs_rebuild = false;
   layerTree->property_trees()->ResetCachedData();
 }
 
 #ifndef NDEBUG
 void PaintArtifactCompositor::showDebugData() {
   LOG(ERROR) << layersAsJSON(LayerTreeIncludesDebugInfo)
                     ->toPrettyJSONString()
                     .utf8()
                     .data();
 }
 #endif
 
 }  // namespace blink