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 134 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;

[David Trainor- moved to gerrit, 2016/01/26 16:34:56]

Does this one need to be changed?  Maybe I'm misreading it.

[sunxd, 2016/01/26 16:39:43]

Sorry this one is changed in the previous CL by me before we realized that it
may break blimp in the future.

 
-  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;
 }