Implement position: sticky updates on compositor

cc/proto/layer_sticky_position_constraint.proto

+// Copyright 2015 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.
+
+syntax = "proto2";
+
+import "rect.proto";
+import "transform.proto";
+
+option optimize_for = LITE_RUNTIME;
+
+package cc.proto;
+
+message LayerStickyPositionConstraint {
+  optional bool is_sticky = 1;
+  optional bool is_anchored_left = 2;
+  optional bool is_anchored_right = 3;
+  optional bool is_anchored_top = 4;
+  optional bool is_anchored_bottom = 5;
+
+  optional float left_offset = 6;
+  optional float right_offset = 7;
+  optional float top_offset = 8;
+  optional float bottom_offset = 9;
+
+  optional Rect scroll_container_relative_sticky_box_rect = 10;
+  optional Rect scroll_container_relative_containing_block_rect = 11;
+};