Add ScrollTree builder and unit test

cc/layers/layer_impl.cc

 // Copyright 2012 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/layers/layer_impl.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <utility>
@@ skipping 70 matching lines @@
       is_affected_by_page_scale_(true),
       was_ever_ready_since_last_transform_animation_(true),
       background_color_(0),
       opacity_(1.0),
       blend_mode_(SkXfermode::kSrcOver_Mode),
       draw_blend_mode_(SkXfermode::kSrcOver_Mode),
       num_descendants_that_draw_content_(0),
       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),
       sorting_context_id_(0),
       current_draw_mode_(DRAW_MODE_NONE),
       element_id_(0),
       mutable_properties_(MutableProperty::kNone),
       force_render_surface_(false),
       frame_timing_requests_dirty_(false),
       visited_(false),
@@ skipping 169 matching lines @@
 void LayerImpl::SetClipTreeIndex(int index) {
   clip_tree_index_ = index;
   SetNeedsPushProperties();
 }
 
 void LayerImpl::SetEffectTreeIndex(int index) {
   effect_tree_index_ = 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
   // a commit to the active tree without a draw.  This only happens in some
   // edge cases during lost context or visibility changes, so don't try to
   // handle preserving these output requests (and their surface).
   if (!copy_requests_.empty()) {
     layer_tree_impl()->RemoveLayerWithCopyOutputRequest(this);
     // Destroying these will abort them.
     copy_requests_.clear();
@@ skipping 360 matching lines @@
   layer->SetScrollCompensationAdjustment(scroll_compensation_adjustment_);
 
   layer->PushScrollOffset(nullptr);
 
   layer->Set3dSortingContextId(sorting_context_id_);
   layer->SetNumDescendantsThatDrawContent(num_descendants_that_draw_content_);
 
   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_);
 
   LayerImpl* scroll_parent = nullptr;
   if (scroll_parent_) {
     scroll_parent = layer->layer_tree_impl()->LayerById(scroll_parent_->id());
     DCHECK(scroll_parent);
   }
 
   layer->SetScrollParent(scroll_parent);
   if (scroll_children_) {
@@ skipping 1280 matching lines @@
            .layer_transforms_should_scale_layer_contents) {
     return default_scale;
   }
 
   gfx::Vector2dF transform_scales = MathUtil::ComputeTransform2dScaleComponents(
       DrawTransform(), default_scale);
   return std::max(transform_scales.x(), transform_scales.y());
 }
 
 }  // namespace cc