Add ScrollTree builder and unit test

cc/proto/property_tree.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 "rectf.proto";
 import "scroll_offset.proto";
 import "transform.proto";
 import "vector2df.proto";
@@ skipping 75 matching lines @@
   optional bool is_drawn = 6;
   optional bool screen_space_opacity_is_animating = 7;
   optional int64 num_copy_requests_in_subtree = 8;
   optional int64 transform_id = 9;
   optional int64 clip_id = 10;
 }
 
 message ScrollNodeData {
   optional bool scrollable = 1;
   optional bool should_scroll_on_main_thread = 2;
-  optional int32 scroll_blocks_on = 3;
-  optional bool contains_non_fast_scrollable_region = 4;
-  optional int64 transform_id = 5;
+  optional bool contains_non_fast_scrollable_region = 3;
+  optional int64 transform_id = 4;
 }
 
 // This defines the proto used for all types of struct TreeNode.
 message TreeNode {
   // The following fields are the base TreeNode properties. This list
   // corresponds to the data members from struct TreeNode.
   optional int64 id = 1;
   optional int64 parent_id = 2;
   optional int64 owner_id = 3;
 
@@ skipping 40 matching lines @@
       [packed = true];
   repeated int64 nodes_affected_by_outer_viewport_bounds_delta = 8
       [packed = true];
 }
 
 // Proto for class PropertyTrees.
 message PropertyTrees {
   optional PropertyTree transform_tree = 1;
   optional PropertyTree effect_tree = 2;
   optional PropertyTree clip_tree = 3;
-  optional PropertyTree scroll_tree = 4;
+  optional PropertyTree scroll_tree = 7;
 
-  optional bool needs_rebuild = 5;
-  optional bool non_root_surfaces_enabled = 6;
-  optional int64 sequence_number = 7;
+  optional bool needs_rebuild = 4;
+  optional bool non_root_surfaces_enabled = 5;
+  optional int64 sequence_number = 6;
 }