Implement position: sticky updates on compositor

cc/trees/transform_node.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_TREES_TRANSFORM_NODE_H_
 #define CC_TREES_TRANSFORM_NODE_H_
 
 #include "cc/base/cc_export.h"
 #include "ui/gfx/geometry/point_f.h"
 #include "ui/gfx/geometry/scroll_offset.h"
@@ skipping 35 matching lines @@
   // directly on the transform/scroll display list items when that's possible,
   // or potentially in a scroll tree.
   //
   // TODO(vollick): will be moved when accelerated effects are implemented.
   gfx::Transform pre_local;
   gfx::Transform local;
   gfx::Transform post_local;
 
   gfx::Transform to_parent;
 
+  // This is the node which defines the sticky position constraints for this
+  // transform node. -1 indicates there are no sticky position constraints.
+  int sticky_position_constraint_id;
+
   // This is the node with respect to which source_offset is defined. This will
   // not be needed once layerization moves to cc, but is needed in order to
   // efficiently update the transform tree for changes to position in the layer
   // tree.
   int source_node_id;
 
   // This id determines which 3d rendering context the node is in. 0 is a
   // special value and indicates that the node is not in any 3d rendering
   // context.
   int sorting_context_id;
@@ skipping 93 matching lines @@
 
   bool operator==(const TransformCachedNodeData& other) const;
 
   void ToProtobuf(proto::TransformCachedNodeData* proto) const;
   void FromProtobuf(const proto::TransformCachedNodeData& proto);
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_TRANSFORM_NODE_H_