[SPv2] Decomposite otherwise-compositable animations that paint nothing.

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 <algorithm>
 #include <memory>
 #include <utility>
 #include "cc/layers/content_layer_client.h"
@@ skipping 571 matching lines @@
   Vector<PaintChunk>::const_iterator cursor =
       paintArtifact.paintChunks().begin();
   layerizeGroup(paintArtifact, pendingLayers, *EffectPaintPropertyNode::root(),
                 cursor);
   DCHECK_EQ(paintArtifact.paintChunks().end(), cursor);
 }
 
 void PaintArtifactCompositor::update(
     const PaintArtifact& paintArtifact,
     RasterInvalidationTrackingMap<const PaintChunk>* rasterChunkInvalidations,
-    bool storeDebugInfo) {
+    bool storeDebugInfo,
+    CompositorElementIdSet& compositedElementIds) {
 #ifndef NDEBUG
   storeDebugInfo = true;
 #endif
 
   DCHECK(m_rootLayer);
 
   cc::LayerTreeHost* layerTreeHost = m_rootLayer->layer_tree_host();
 
   // The tree will be null after detaching and this update can be ignored.
   // See: WebViewImpl::detachPaintArtifactCompositor().
@@ skipping 24 matching lines @@
 
     const auto* transform = pendingLayer.propertyTreeState.transform();
     int transformId =
         propertyTreeManager.ensureCompositorTransformNode(transform);
     int clipId = propertyTreeManager.ensureCompositorClipNode(
         pendingLayer.propertyTreeState.clip());
     int effectId = propertyTreeManager.switchToEffectNode(
         *pendingLayer.propertyTreeState.effect());
 
     layer->set_offset_to_transform_parent(layerOffset);
-    layer->SetElementId(pendingLayer.propertyTreeState.compositorElementId());
+    CompositorElementId elementId =
+        pendingLayer.propertyTreeState.compositorElementId();
+    if (elementId) {
+      layer->SetElementId(elementId);
+      compositedElementIds.insert(elementId);
+    }
 
     m_rootLayer->AddChild(layer);
     layer->set_property_tree_sequence_number(sPropertyTreeSequenceNumber);
     layer->SetTransformTreeIndex(transformId);
     layer->SetClipTreeIndex(clipId);
     layer->SetEffectTreeIndex(effectId);
     propertyTreeManager.updateLayerScrollMapping(layer.get(), transform);
 
     layer->SetShouldCheckBackfaceVisibility(pendingLayer.backfaceHidden);
 
@@ skipping 15 matching lines @@
 #ifndef NDEBUG
 void PaintArtifactCompositor::showDebugData() {
   LOG(ERROR) << layersAsJSON(LayerTreeIncludesDebugInfo)
                     ->toPrettyJSONString()
                     .utf8()
                     .data();
 }
 #endif
 
 }  // namespace blink