Ensure PaintArtifactCompositor assigns a scroll tree index to all cc layers

third_party/WebKit/Source/platform/graphics/paint/TransformPaintPropertyNode.cpp

 // Copyright 2016 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/paint/TransformPaintPropertyNode.h"
 
 #include "platform/graphics/paint/PropertyTreeState.h"
 
 namespace blink {
 
 // The root of the transform tree. The root transform node references the root
 // scroll node.
 TransformPaintPropertyNode* TransformPaintPropertyNode::root() {
   DEFINE_STATIC_REF(TransformPaintPropertyNode, root,
                     adoptRef(new TransformPaintPropertyNode(
                         nullptr, TransformationMatrix(), FloatPoint3D(), false,
                         0, CompositingReasonNone, CompositorElementId(),
                         ScrollPaintPropertyNode::root())));
   return root;
 }
 
+const ScrollPaintPropertyNode*
+TransformPaintPropertyNode::findEnclosingScrollNode() const {
+  if (m_scroll)
+    return m_scroll.get();
+
+  for (const auto* ancestor = parent(); ancestor;
+       ancestor = ancestor->parent()) {
+    if (const auto* scrollNode = ancestor->scrollNode())
+      return scrollNode;
+  }
+  // The root transform node references the root scroll node so a scroll node
+  // should always exist.
+  NOTREACHED();
+  return nullptr;
+}
+
 String TransformPaintPropertyNode::toString() const {
   auto transform = String::format(
       "parent=%p transform=%s origin=%s flattensInheritedTransform=%s "
       "renderingContextId=%x directCompositingReasons=%s "
       "compositorElementId=(%d, %d)",
       m_parent.get(), m_matrix.toString().ascii().data(),
       m_origin.toString().ascii().data(),
       m_flattensInheritedTransform ? "yes" : "no", m_renderingContextId,
       compositingReasonsAsString(m_directCompositingReasons).ascii().data(),
       m_compositorElementId.primaryId, m_compositorElementId.secondaryId);
   if (m_scroll)
     return transform + " scroll=" + m_scroll->toString();
   return transform;
 }
 
 #if DCHECK_IS_ON()
 
 String TransformPaintPropertyNode::toTreeString() const {
   return blink::PropertyTreeStatePrinter<blink::TransformPaintPropertyNode>()
       .pathAsString(this);
 }
 
 #endif
 
 }  // namespace blink