Set layer element id when building layers in PaintArtifactCompositor.

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

Index: third_party/WebKit/Source/platform/graphics/paint/PropertyTreeState.cpp
diff --git a/third_party/WebKit/Source/platform/graphics/paint/PropertyTreeState.cpp b/third_party/WebKit/Source/platform/graphics/paint/PropertyTreeState.cpp
index 852b5243edb25b2eb73911ebda9fa37f3370faaf..c350165afdf9f5b5e9b38841ea35bf4682d9d76d 100644
--- a/third_party/WebKit/Source/platform/graphics/paint/PropertyTreeState.cpp
+++ b/third_party/WebKit/Source/platform/graphics/paint/PropertyTreeState.cpp
@@ -29,6 +29,36 @@ bool isAncestorOf(const PropertyNode* ancestor, const PropertyNode* child) {
   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.