Compute and include the offset of the sticky box to its enclosing composited layer.

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 b575a23aa26bf1978baaf11a462c0818222e7af4..51a4da0c0b2853b00aded94e833f2dcfcf3f6b28 100644
--- a/cc/layers/layer_sticky_position_constraint.h
+++ b/cc/layers/layer_sticky_position_constraint.h
@@ -32,6 +32,11 @@ struct CC_EXPORT LayerStickyPositionConstraint {
   float top_offset;
   float bottom_offset;
 
+  // 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;
+
   // 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