Add ScrollTree builder and unit test

cc/layers/layer_impl.h

 // Copyright 2011 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.
 
 #ifndef CC_LAYERS_LAYER_IMPL_H_
 #define CC_LAYERS_LAYER_IMPL_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 152 matching lines @@
 
   void SetTransformTreeIndex(int index);
   int transform_tree_index() const { return transform_tree_index_; }
 
   void SetClipTreeIndex(int index);
   int clip_tree_index() const { return clip_tree_index_; }
 
   void SetEffectTreeIndex(int index);
   int effect_tree_index() const { return effect_tree_index_; }
 
+  void SetScrollTreeIndex(int index);
+  int scroll_tree_index() const { return scroll_tree_index_; }
+
   void set_offset_to_transform_parent(const gfx::Vector2dF& offset) {
     offset_to_transform_parent_ = offset;
     SetNeedsPushProperties();
   }
   gfx::Vector2dF offset_to_transform_parent() const {
     return offset_to_transform_parent_;
   }
 
   const gfx::Rect& visible_rect_from_property_trees() const {
     return visible_rect_from_property_trees_;
@@ skipping 644 matching lines @@
 
   gfx::Vector2dF scroll_compensation_adjustment_;
 
   int num_descendants_that_draw_content_;
 
   gfx::Rect visible_rect_from_property_trees_;
   gfx::Rect clip_rect_in_target_space_from_property_trees_;
   int transform_tree_index_;
   int effect_tree_index_;
   int clip_tree_index_;
+  int scroll_tree_index_;
 
   // The global depth value of the center of the layer. This value is used
   // to sort layers from back to front.
   float draw_depth_;
 
   FilterOperations filters_;
   FilterOperations background_filters_;
 
  protected:
   friend class TreeSynchronizer;
@@ skipping 49 matching lines @@
   bool layer_or_descendant_has_input_handler_;
   bool sorted_for_recursion_;
   bool is_hidden_from_property_trees_;
 
   DISALLOW_COPY_AND_ASSIGN(LayerImpl);
 };
 
 }  // namespace cc
 
 #endif  // CC_LAYERS_LAYER_IMPL_H_