Add ScrollTree builder and unit test

cc/trees/property_tree_unittest.cc

Index: cc/trees/property_tree_unittest.cc
diff --git a/cc/trees/property_tree_unittest.cc b/cc/trees/property_tree_unittest.cc
index ac3e067fad84f71ec55be220d1f671ee2beb2e8e..42f0fdebfa23afa868aed6a2eea386091714a3ed 100644
--- a/cc/trees/property_tree_unittest.cc
+++ b/cc/trees/property_tree_unittest.cc
@@ -267,6 +267,9 @@ TEST(PropertyTreeSerializationTest, ScrollTreeSerialization) {
   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;