WIP: Construct SPV2's scroll paint property tree

cc/trees/scroll_node.h

Index: cc/trees/scroll_node.h
diff --git a/cc/trees/scroll_node.h b/cc/trees/scroll_node.h
index 745ee69214c25d8bdf95c425fd97b4bcaa54af05..041c48de6b95d9738954cbccc1e2794ccb194672 100644
--- a/cc/trees/scroll_node.h
+++ b/cc/trees/scroll_node.h
@@ -27,22 +27,39 @@ struct CC_EXPORT ScrollNode {
 
   int id;
   int parent_id;
+
+  // The layer id that corresponds to the layer contents that are scrolled.
   int owner_id;
 
+  // This is used to delineate a subtree that should not be scrolled. For
+  // example, when there is a layer that is not scrollable itself but is inside
+  // a scrolling layer.
   bool scrollable;
+
   uint32_t main_thread_scrolling_reasons;
   bool contains_non_fast_scrollable_region;
+
+  // Size of the clipped area, not including non-overlay scrollbars. Overlay
+  // scrollbars do not affect the clipped area.
   gfx::Size scroll_clip_layer_bounds;
+
+  // Bounds of the overflow scrolling area.
   gfx::Size bounds;
+
   bool max_scroll_offset_affected_by_page_scale;
   bool is_inner_viewport_scroll_layer;
   bool is_outer_viewport_scroll_layer;
+
+  // This offset is used when |scrollable| is false and there isn't a transform
+  // already present that covers this offset.
   gfx::Vector2dF offset_to_transform_parent;
+
   bool should_flatten;
   bool user_scrollable_horizontal;
   bool user_scrollable_vertical;
   ElementId element_id;
   int transform_id;
+
   // Number of drawn layers pointing to this node or any of its descendants.
   int num_drawn_descendants;