[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 906b3ed09c5693e93521154066216337518a8569..4a6ab4140f160d5f51bb85a48215a9a03c369962 100644
--- a/third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.cpp
+++ b/third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.cpp
@@ -596,7 +596,8 @@ void PaintArtifactCompositor::update(
     const PaintArtifact& paintArtifact,
     RasterInvalidationTrackingMap<const PaintChunk>* rasterChunkInvalidations,
     bool storeDebugInfo,
-    GeometryMapper& geometryMapper) {
+    GeometryMapper& geometryMapper,
+    CompositorElementIdSet& compositedElementIds) {
 #ifndef NDEBUG
   storeDebugInfo = true;
 #endif
@@ -641,7 +642,12 @@ 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);
+      compositedElementIds.insert(elementId);
+    }
 
     m_rootLayer->AddChild(layer);
     layer->set_property_tree_sequence_number(sPropertyTreeSequenceNumber);