Add ScrollTree builder and unit test

cc/layers/layer.h

 // Copyright 2010 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_H_
 #define CC_LAYERS_LAYER_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 464 matching lines @@
 
   void SetTransformTreeIndex(int index);
   int transform_tree_index() const;
 
   void SetClipTreeIndex(int index);
   int clip_tree_index() const;
 
   void SetEffectTreeIndex(int index);
   int effect_tree_index() const;
 
+  void SetScrollTreeIndex(int index);
+  int scroll_tree_index() const;
+
   void set_offset_to_transform_parent(gfx::Vector2dF offset) {
     if (offset_to_transform_parent_ == offset)
       return;
     offset_to_transform_parent_ = offset;
     SetNeedsPushProperties();
   }
   gfx::Vector2dF offset_to_transform_parent() const {
     return offset_to_transform_parent_;
   }
 
@@ skipping 219 matching lines @@
   gfx::ScrollOffset scroll_offset_;
   gfx::Vector2dF scroll_compensation_adjustment_;
   // This variable indicates which ancestor layer (if any) whose size,
   // transformed relative to this layer, defines the maximum scroll offset for
   // this layer.
   int scroll_clip_layer_id_;
   int num_descendants_that_draw_content_;
   int transform_tree_index_;
   int effect_tree_index_;
   int clip_tree_index_;
+  int scroll_tree_index_;
   int property_tree_sequence_number_;
   uint64_t element_id_;
   uint32_t mutable_properties_;
   gfx::Vector2dF offset_to_transform_parent_;
   uint32_t main_thread_scrolling_reasons_;
   bool should_flatten_transform_from_property_tree_ : 1;
   bool have_wheel_event_handlers_ : 1;
   bool have_scroll_event_handlers_ : 1;
   bool user_scrollable_horizontal_ : 1;
   bool user_scrollable_vertical_ : 1;
@@ skipping 59 matching lines @@
   std::vector<FrameTimingRequest> frame_timing_requests_;
   bool frame_timing_requests_dirty_;
   bool is_hidden_from_property_trees_;
 
   DISALLOW_COPY_AND_ASSIGN(Layer);
 };
 
 }  // namespace cc
 
 #endif  // CC_LAYERS_LAYER_H_