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

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 a3588a3c5881986c159ee6ac93d7a53a135f030b..68bf7c06b739495051575df81f6baef0f78af4b1 100644
--- a/third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.cpp
+++ b/third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.cpp
@@ -452,7 +452,8 @@ void PaintArtifactCompositor::update(
     const PaintArtifact& paintArtifact,
     RasterInvalidationTrackingMap<const PaintChunk>* rasterChunkInvalidations,
     bool storeDebugInfo,
-    GeometryMapper& geometryMapper) {
+    GeometryMapper& geometryMapper,
+    CompositorElementIdSet& compositedAnimationElementIds) {
 #ifndef NDEBUG
   storeDebugInfo = true;
 #endif
@@ -497,7 +498,12 @@ void PaintArtifactCompositor::update(
         *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);
+      compositedAnimationElementIds.insert(elementId);
+    }
 
     m_rootLayer->AddChild(layer);
     layer->set_property_tree_sequence_number(sPropertyTreeSequenceNumber);