Handle nested position:sticky elements correctly (compositor)

cc/trees/property_tree.h

Index: cc/trees/property_tree.h
diff --git a/cc/trees/property_tree.h b/cc/trees/property_tree.h
index 8c9d158a7bc28ee99741741d115eba725fbe0505..ae94391d483bfeee1aec12bbe266adc2afb15d43 100644
--- a/cc/trees/property_tree.h
+++ b/cc/trees/property_tree.h
@@ -114,7 +114,17 @@ struct StickyPositionNodeData {
   // by computing the additional offset necessary to keep the element stuck.
   gfx::Vector2dF main_thread_offset;
 
-  StickyPositionNodeData() : scroll_ancestor(-1) {}
+  // In order to properly compute the sticky offset, we need to know if we have
+  // any sticky ancestors both between ourselves and our containing block and
+  // between our containing block and the viewport. These ancestors are then
+  // used to correct the constraining rect locations.
+  int nearest_node_shifting_sticky_box;
+  int nearest_node_shifting_containing_block;
+
+  StickyPositionNodeData()
+      : scroll_ancestor(-1),
+        nearest_node_shifting_sticky_box(-1),
+        nearest_node_shifting_containing_block(-1) {}
 };
 
 class CC_EXPORT TransformTree final : public PropertyTree<TransformNode> {