Set layer element id when building layers in PaintArtifactCompositor.

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

Index: third_party/WebKit/Source/platform/graphics/paint/PropertyTreeState.h
diff --git a/third_party/WebKit/Source/platform/graphics/paint/PropertyTreeState.h b/third_party/WebKit/Source/platform/graphics/paint/PropertyTreeState.h
index 19555592996153b78883df9520f1af9cc9f6b1c2..7bcf40dcc8b13cd320a46b07f80ac56b58213137 100644
--- a/third_party/WebKit/Source/platform/graphics/paint/PropertyTreeState.h
+++ b/third_party/WebKit/Source/platform/graphics/paint/PropertyTreeState.h
@@ -68,6 +68,11 @@ class PLATFORM_EXPORT PropertyTreeState {
     m_scroll = std::move(node);
   }
 
+  // Returns the compositor element id, if any, for this property state. If none
+  // of the scroll, effect or transform nodes for this state have a compositor
+  // element id then a default instance is returned.
+  const CompositorElementId compositorElementId() const;

[wkorman, 2017/01/20 00:56:38]

We could put this logic in PaintArtifactCompositor as a static method but seemed
cleaner this way, open to thoughts though.

+
   enum InnermostNode {
     None,  // None means that all nodes are their root values
     Transform,