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..052f73ea10fa10519ccb4506c3e75dfcd3aab694 100644
--- a/cc/trees/scroll_node.h
+++ b/cc/trees/scroll_node.h
@@ -27,17 +27,33 @@ struct CC_EXPORT ScrollNode {
 
   int id;
   int parent_id;
+
+  // The layer id that corresponds to the layer contents that are scrolled.

[ajuma, 2016/09/07 14:19:23]

Please also mention that, unlike "id" which can change each time we rebuild
property trees, this id is stable.

   int owner_id;

[chrishtr, 2016/09/06 23:40:16]

Rename to owning_layer_id?

[ajuma, 2016/09/07 14:19:23]

Since changing the scroll offset stored at this node can potentially move many
layers, and since there's no hierarchical relationship between these layers in
SPv2, or even now on the compositor thread, if we rename this I'd prefer that
the name emphasizes that this id is stable rather than that it happens to match
a layer id right now (we're working towards removing the assumption that
property tree nodes have "owning" layers). Also whatever terminology we end up
using here should be consistent across node types.

[pdr., 2016/09/07 17:58:11]

It looks like the owner_id name is used for many other trees too. I think it
could make sense to rename them all to stable_id (or similar), but in a separate
patch. Are you okay with deferring this to a later patch Chrishtr?

I've updated the comment to mention that this id is stable.

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

cool.

 
+  // This is used to delineate a subtree that should not be scrolled. For

[ajuma, 2016/09/07 14:19:23]

"should not be scrolled independently" maybe?

[pdr., 2016/09/07 17:58:11]

I changed this line to "This is used for subtrees that should not be scrolled
independently." which is cleaner and simpler.

+  // 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

[ajuma, 2016/09/07 14:19:23]

"transform node"

[pdr., 2016/09/07 17:58:11]

Done

+  // already present that covers this offset.
   gfx::Vector2dF offset_to_transform_parent;
+
   bool should_flatten;
   bool user_scrollable_horizontal;
   bool user_scrollable_vertical;