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..dd0e52c4fd5797df3be0d16b1754374d2862467a 100644
--- a/cc/layers/layer_sticky_position_constraint.h
+++ b/cc/layers/layer_sticky_position_constraint.h
@@ -7,7 +7,9 @@
 
 #include "cc/cc_export.h"
 
-#include "ui/gfx/geometry/rect.h"
+#include "ui/gfx/geometry/point_f.h"
+#include "ui/gfx/geometry/rect_f.h"
+#include "ui/gfx/geometry/vector2d_f.h"
 
 namespace cc {
 
@@ -31,18 +33,28 @@ struct CC_EXPORT LayerStickyPositionConstraint {
   // The layout offset of the sticky box relative to its containing layer.
   // This is used to detect the sticky offset the main thread has applied
   // to the layer.
-  gfx::Point parent_relative_sticky_box_offset;
+  gfx::PointF parent_relative_sticky_box_offset;
 
   // The rectangle corresponding to original layout position of the sticky box
   // relative to the scroll ancestor. The sticky box is only offset once the
   // scroll has passed its initial position (e.g. top_offset will only push
   // the element down from its original position).
-  gfx::Rect scroll_container_relative_sticky_box_rect;
+  gfx::RectF scroll_container_relative_sticky_box_rect;
 
   // The layout rectangle of the sticky box's containing block relative to the
   // scroll ancestor. The sticky box is only moved as far as its containing
   // block boundary.
-  gfx::Rect scroll_container_relative_containing_block_rect;
+  gfx::RectF 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;