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..ea89d4d3616b1f044f770d399820fb0306a396a6 100644
--- a/cc/trees/scroll_node.h
+++ b/cc/trees/scroll_node.h
@@ -27,22 +27,40 @@ struct CC_EXPORT ScrollNode {
 
   int id;
   int parent_id;
+
+  // This is the layer id that corresponds to the layer that is scrolled. In
+  // blink this corresponds to the scrolling content layer.
+  // TODO(pdr): Clarify this with Ali and Xianda.
   int owner_id;
 
+  // This could be false when you have a layer that is not scrollable itself but
+  // is inside a scroller.
+  // TODO(pdr): Clarify this with Ali and Xianda.
   bool scrollable;
+
   uint32_t main_thread_scrolling_reasons;
   bool contains_non_fast_scrollable_region;
+
+  // Size of the clipped area, not including scrollbars.

[chrishtr, 2016/09/02 18:04:37]

Except for overlay scrollbars.

   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;
+
+  // TODO(pdr): Clarify this with Ali and Xianda.
+  // Why isn't this == TransformToOffsetParent(Transform(transform_id)).

[chrishtr, 2016/09/02 18:04:37]

Used if |scrollable| is false, and there doesn't happen to be a transform node
to be equal to.

   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;