Handle nested position:sticky elements correctly (compositor)

cc/layers/layer_sticky_position_constraint.h

Index: cc/layers/layer_sticky_position_constraint.h
diff --git a/cc/layers/layer_sticky_position_constraint.h b/cc/layers/layer_sticky_position_constraint.h
index 2082a2b2652d37983475f12728376f5d83110d54..adf053528f541aa570e23dd4ee477f006500e9be 100644
--- a/cc/layers/layer_sticky_position_constraint.h
+++ b/cc/layers/layer_sticky_position_constraint.h
@@ -7,6 +7,7 @@
 
 #include "cc/cc_export.h"
 
+#include "ui/gfx/geometry/point.h"
 #include "ui/gfx/geometry/rect.h"
 
 namespace cc {
@@ -44,6 +45,16 @@ struct CC_EXPORT LayerStickyPositionConstraint {
   // block boundary.
   gfx::Rect scroll_container_relative_containing_block_rect;
 
+  // The nearest ancestor sticky layer ids that affect the sticky box constraint
+  // rect and the containing block constraint rect respectively. If no such
+  // layer exists, these are set to Layer::INVALID_ID.
+  int nearest_layer_shifting_sticky_box;
+  int nearest_layer_shifting_containing_block;
+
+  // Returns the nearest sticky ancestor layer, or Layer::INVALID_ID if no such
+  // layer exists.
+  int NearestStickyAncestor();
+
   bool operator==(const LayerStickyPositionConstraint&) const;
   bool operator!=(const LayerStickyPositionConstraint&) const;
 };