Add ScrollTree builder and unit test

cc/layers/layer_impl.cc

Index: cc/layers/layer_impl.cc
diff --git a/cc/layers/layer_impl.cc b/cc/layers/layer_impl.cc
index 540de09d49b251b5527c86288574a24212617099..95ad8019a92e94964d130842ef1d67524026d44c 100644
--- a/cc/layers/layer_impl.cc
+++ b/cc/layers/layer_impl.cc
@@ -88,6 +88,7 @@ LayerImpl::LayerImpl(LayerTreeImpl* tree_impl,
       transform_tree_index_(-1),
       effect_tree_index_(-1),
       clip_tree_index_(-1),
+      scroll_tree_index_(-1),
       draw_depth_(0.f),
       needs_push_properties_(false),
       num_dependents_need_push_properties_(0),
@@ -278,6 +279,11 @@ void LayerImpl::SetEffectTreeIndex(int index) {
   SetNeedsPushProperties();
 }
 
+void LayerImpl::SetScrollTreeIndex(int index) {
+  scroll_tree_index_ = index;
+  SetNeedsPushProperties();
+}
+
 void LayerImpl::PassCopyRequests(
     std::vector<scoped_ptr<CopyOutputRequest>>* requests) {
   // In the case that a layer still has a copy request, this means that there's
@@ -658,6 +664,7 @@ void LayerImpl::PushPropertiesTo(LayerImpl* layer) {
   layer->SetTransformTreeIndex(transform_tree_index_);
   layer->SetClipTreeIndex(clip_tree_index_);
   layer->SetEffectTreeIndex(effect_tree_index_);
+  layer->SetScrollTreeIndex(scroll_tree_index_);
   layer->set_offset_to_transform_parent(offset_to_transform_parent_);
   layer->set_is_hidden_from_property_trees(is_hidden_from_property_trees_);