Keep track of fixed positioning in accessibility tree.

ui/accessibility/ax_relative_bounds.h

Index: ui/accessibility/ax_relative_bounds.h
diff --git a/ui/accessibility/ax_relative_bounds.h b/ui/accessibility/ax_relative_bounds.h
index 478275a9ebbca114d515c53b134d2f83293409a4..88f873f016d46bec7284ec7982cd29c24c0b8631 100644
--- a/ui/accessibility/ax_relative_bounds.h
+++ b/ui/accessibility/ax_relative_bounds.h
@@ -21,8 +21,8 @@ namespace ui {
 // This is an efficient, compact, serializable representation of a node's
 // bounding box that requires minimal changes to the tree when layers are
 // moved or scrolled. Computing the absolute bounding box of a node requires
-// walking up the tree and applying node offsets and transforms until reaching
-// the top.
+// walking up the tree and applying node offsets, scroll positions, and
+// transforms until reaching the top.
 //
 // If the offset container id is valid, the bounds are relative
 // to the node with that offset container id.
@@ -30,6 +30,13 @@ namespace ui {
 // Otherwise, for a node other than the root, the bounds are relative to
 // the root of the tree, and for the root of a tree, the bounds are relative
 // to its immediate containing node.
+//
+// If |is_fixed_positioned| is true, the scroll offsets of the container
+// node should be ignored.
+//
+// The canonical implementation of converting a node's bounding box
+// from local coordinates to tree coordinates is AXTree::GetTreeBounds().
+// Always try to use that rather than reimplementing it.
 struct AX_EXPORT AXRelativeBounds {
   AXRelativeBounds();
   virtual ~AXRelativeBounds();
@@ -54,6 +61,9 @@ struct AX_EXPORT AXRelativeBounds {
   // constructor both make a duplicate of the owned pointer, so it acts more
   // like a member than a pointer.
   std::unique_ptr<gfx::Transform> transform;
+
+  // If true, the container's scroll position should be ignored.
+  bool is_fixed_positioned;
 };
 
 }  // namespace ui