Set layer element id when building layers in PaintArtifactCompositor.

third_party/WebKit/Source/platform/graphics/paint/PropertyTreeState.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/PropertyTreeState.h"
 
 #include "platform/graphics/paint/GeometryMapper.h"
 
 namespace blink {
 
@@ skipping 11 matching lines @@
 }
 
 template <typename PropertyNode>
 bool isAncestorOf(const PropertyNode* ancestor, const PropertyNode* child) {
   while (child && child != ancestor) {
     child = child->parent();
   }
   return child == ancestor;
 }
 
+const CompositorElementId PropertyTreeState::compositorElementId() const {
+  // Zero or more of the scroll, effect or transform nodes could have a
+  // compositor element id. The order doesn't matter as the element id should be
+  // the same on all that have a non-default CompositorElementId.

[ajuma, 2017/01/20 15:07:46]

Is it worth DCHECK-ing that this holds?

[wkorman, 2017/01/20 22:13:56]

Done.

+  if (effect()->compositorElementId())
+    return effect()->compositorElementId();
+  if (scroll()->compositorElementId())
+    return scroll()->compositorElementId();
+  if (transform()->compositorElementId())
+    return transform()->compositorElementId();
+  return CompositorElementId();
+}
+
 PropertyTreeState::InnermostNode PropertyTreeState::innermostNode() const {
   // TODO(chrishtr): this is very inefficient when innermostNode() is called
   // repeatedly.
   bool clipTransformStrictAncestorOfTransform =
       m_clip->localTransformSpace() != m_transform.get() &&
       isAncestorOf<TransformPaintPropertyNode>(m_clip->localTransformSpace(),
                                                m_transform.get());
   bool effectTransformStrictAncestorOfTransform =
       m_effect->localTransformSpace() != m_transform.get() &&
       isAncestorOf<TransformPaintPropertyNode>(m_effect->localTransformSpace(),
@@ skipping 25 matching lines @@
     case PropertyTreeState::Effect:
       m_properties.setEffect(m_properties.effect()->parent());
       return &m_properties;
     case PropertyTreeState::None:
       return nullptr;
   }
   return nullptr;
 }
 
 }  // namespace blink