Add comments describing the cc ScrollNode members

cc/trees/scroll_node.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_TREES_SCROLL_NODE_H_
 #define CC_TREES_SCROLL_NODE_H_
 
 #include "cc/base/cc_export.h"
 #include "cc/output/filter_operations.h"
 #include "ui/gfx/geometry/size.h"
 
 namespace base {
 namespace trace_event {
 class TracedValue;
 }  // namespace trace_event
 }  // namespace base
 
 namespace cc {
 
 namespace proto {
 class TreeNode;
 }  // namespace proto
 
 struct CC_EXPORT ScrollNode {
   ScrollNode();
   ScrollNode(const ScrollNode& other);
 
   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;
   ElementId element_id;
   int transform_id;
   // Number of drawn layers pointing to this node or any of its descendants.
   int num_drawn_descendants;
 
   bool operator==(const ScrollNode& other) const;
 
   void ToProtobuf(proto::TreeNode* proto) const;
   void FromProtobuf(const proto::TreeNode& proto);
   void AsValueInto(base::trace_event::TracedValue* value) const;
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_SCROLL_NODE_H_