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.
+#if DCHECK_IS_ON()
+  CompositorElementId expectedElementId;
+  if (CompositorElementId actualElementId = effect()->compositorElementId()) {
+    expectedElementId = actualElementId;
+  }
+  if (CompositorElementId actualElementId = scroll()->compositorElementId()) {
+    if (!expectedElementId)
+      expectedElementId = actualElementId;
+    else
+      DCHECK_EQ(expectedElementId, actualElementId);
+  }
+  if (CompositorElementId actualElementId =
+          transform()->compositorElementId()) {
+    if (expectedElementId)
+      DCHECK_EQ(expectedElementId, actualElementId);
+  }
+#endif
+  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 34 matching lines @@
 #if DCHECK_IS_ON()
 
 String PropertyTreeState::toTreeString() const {
   return transform()->toTreeString() + "\n" + clip()->toTreeString() + "\n" +
          effect()->toTreeString() + "\n" + scroll()->toTreeString();
 }
 
 #endif
 
 }  // namespace blink