Add comments describing the cc ScrollNode members

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..21ac7038ade01c5d52af57560da22375c7142c2d 100644
--- a/cc/trees/scroll_node.h
+++ b/cc/trees/scroll_node.h
@@ -27,17 +27,35 @@ struct CC_EXPORT ScrollNode {
 
   int id;
   int parent_id;
+
+  // The layer id that corresponds to the layer contents that are scrolled.
+  // Unlike |id| which can change each time property trees are rebuilt, this id
+  // is stable.

[chrishtr, 2016/09/07 17:59:45]

"stable across property tree rebuilds". It's not stable against mutations in
Blink which require
rebuilding the composited layer tree.

[pdr., 2016/09/07 18:02:48]

Hmm... I see what you mean. This is tough because we don't have a concrete
definition of stability yet.

Do you have a suggestion for how to phrase this better?

[chrishtr, 2016/09/07 18:05:30]

Stable across frames that don't include DOM mutations? Or maybe just explictly
"stable across frames that don't change the composited layer list"

   int owner_id;
 
+  // This is used for subtrees that should not be scrolled independently. 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
+  // node already present that covers this offset.
   gfx::Vector2dF offset_to_transform_parent;
+
   bool should_flatten;
   bool user_scrollable_horizontal;
   bool user_scrollable_vertical;