Slightly refactor StickyPositionScrollingConstraints API and add documentation

third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp

Index: third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
diff --git a/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp b/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
index 7c24cc5fa4fd727da6244c48a09fb8f678d20ab1..0a6172024b47238d204fce6c0df6111b43a8000c 100644
--- a/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
+++ b/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
@@ -331,25 +331,22 @@ void CompositedLayerMapping::UpdateStickyConstraints(
   // TODO(smcgruer): Until http://crbug.com/702229 is fixed, the nearest
   // sticky layers may not be composited and we may incorrectly end up with
   // invalid layer IDs.
-  LayoutBoxModelObject* sticky_box_shifting_ancestor =
-      constraints.NearestStickyBoxShiftingStickyBox();
+  PaintLayer* sticky_box_shifting_ancestor =
+      constraints.NearestStickyLayerShiftingStickyBox();
   if (sticky_box_shifting_ancestor &&
-      sticky_box_shifting_ancestor->Layer()->GetCompositedLayerMapping()) {
+      sticky_box_shifting_ancestor->GetCompositedLayerMapping()) {
     web_constraint.nearest_layer_shifting_sticky_box =
-        sticky_box_shifting_ancestor->Layer()
-            ->GetCompositedLayerMapping()
+        sticky_box_shifting_ancestor->GetCompositedLayerMapping()
             ->MainGraphicsLayer()
             ->PlatformLayer()
             ->Id();
   }
-  LayoutBoxModelObject* containing_block_shifting_ancestor =
-      constraints.NearestStickyBoxShiftingContainingBlock();
+  PaintLayer* containing_block_shifting_ancestor =
+      constraints.NearestStickyLayerShiftingContainingBlock();
   if (containing_block_shifting_ancestor &&
-      containing_block_shifting_ancestor->Layer()
-          ->GetCompositedLayerMapping()) {
+      containing_block_shifting_ancestor->GetCompositedLayerMapping()) {
     web_constraint.nearest_layer_shifting_containing_block =
-        containing_block_shifting_ancestor->Layer()
-            ->GetCompositedLayerMapping()
+        containing_block_shifting_ancestor->GetCompositedLayerMapping()
             ->MainGraphicsLayer()
             ->PlatformLayer()
             ->Id();