Add ScrollTree builder and unit test

cc/trees/property_tree_unittest.cc

 // Copyright 2014 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.
 
 #include "cc/trees/property_tree.h"
 
 #include "cc/proto/property_tree.pb.h"
 #include "cc/test/geometry_test_utils.h"
 #include "cc/trees/draw_property_utils.h"
 #include "testing/gtest/include/gtest/gtest.h"
@@ skipping 249 matching lines @@
   ScrollTree original;
   ScrollNode& root = *original.Node(0);
   root.data.transform_id = 2;
   ScrollNode second;
   second.data.transform_id = 4;
   second.data.scrollable = true;
   ScrollNode third;
   third.data.transform_id = 5;
   third.data.contains_non_fast_scrollable_region = true;
 
+  original.Insert(second, 0);
+  original.Insert(third, 1);
+
   proto::PropertyTree proto;
   original.ToProtobuf(&proto);
   ScrollTree result;
   result.FromProtobuf(proto);
 
   EXPECT_EQ(original, result);
 }
 
 TEST(PropertyTreeSerializationTest, PropertyTrees) {
   PropertyTrees original;
@@ skipping 783 matching lines @@
 
 DIRECT_AND_SERIALIZED_PROPERTY_TREE_TEST_F(
     PropertyTreeTestSingularTransformSnapTest);
 
 #undef DIRECT_AND_SERIALIZED_PROPERTY_TREE_TEST_F
 #undef SERIALIZED_PROPERTY_TREE_TEST_F
 #undef DIRECT_PROPERTY_TREE_TEST_F
 
 }  // namespace
 }  // namespace cc