WIP: Construct SPV2's scroll paint property tree

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;
+
+  // 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;
 
   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_