Refactor scroll chaining to use ScrollNodes without going through LayerImpl

cc/trees/layer_tree_host_impl.cc

 // 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.
 
 #include "cc/trees/layer_tree_host_impl.h"
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 2992 matching lines @@
     return ScrollNodeWithViewportSpaceDelta(
         scroll_node, gfx::PointF(viewport_point), delta, scroll_tree);
   }
   float scale_factor = active_tree()->current_page_scale_factor();
   return ScrollNodeWithLocalDelta(scroll_node, delta, scale_factor,
                                   active_tree());
 }
 
 void LayerTreeHostImpl::ApplyScroll(ScrollNode* scroll_node,
                                     ScrollState* scroll_state) {
-  DCHECK(scroll_state);
+  DCHECK(scroll_node && scroll_state);
   gfx::Point viewport_point(scroll_state->position_x(),
                             scroll_state->position_y());
   const gfx::Vector2dF delta(scroll_state->delta_x(), scroll_state->delta_y());
   gfx::Vector2dF applied_delta;
   gfx::Vector2dF delta_applied_to_content;
   // TODO(tdresser): Use a more rational epsilon. See crbug.com/510550 for
   // details.
   const float kEpsilon = 0.1f;
 
   bool is_viewport_scroll_layer =
@@ skipping 57 matching lines @@
   scroll_state->ConsumeDelta(applied_delta.x(), applied_delta.y());
 
   scroll_state->set_current_native_scrolling_node(scroll_node);
 }
 
 void LayerTreeHostImpl::DistributeScrollDelta(ScrollState* scroll_state) {
   // TODO(majidvp): in Blink we compute scroll chain only at scroll begin which
   // is not the case here. We eventually want to have the same behaviour on both
   // sides but it may become a non issue if we get rid of scroll chaining (see
   // crbug.com/526462)
-  std::list<const ScrollNode*> current_scroll_chain;
+  std::list<ScrollNode*> current_scroll_chain;
   ScrollTree& scroll_tree = active_tree_->property_trees()->scroll_tree;
   ScrollNode* scroll_node = scroll_tree.CurrentlyScrollingNode();
   ScrollNode* viewport_scroll_node =
       viewport()->MainScrollLayer()
           ? scroll_tree.Node(viewport()->MainScrollLayer()->scroll_tree_index())
           : nullptr;
   if (scroll_node) {
     // TODO(bokan): The loop checks for a null parent but don't we still want to
     // distribute to the root scroll node?
     for (; scroll_tree.parent(scroll_node);
          scroll_node = scroll_tree.parent(scroll_node)) {
       if (scroll_node == viewport_scroll_node) {
         // Don't chain scrolls past the outer viewport scroll layer. Once we
         // reach that, we should scroll the viewport which is represented by the
         // main viewport scroll layer.
         DCHECK(viewport_scroll_node);
         current_scroll_chain.push_front(viewport_scroll_node);
         break;
       }
 
       if (!scroll_node->scrollable)
         continue;
 
       current_scroll_chain.push_front(scroll_node);
     }
   }
-  scroll_state->set_scroll_chain_and_layer_tree(current_scroll_chain,
+  scroll_state->set_scroll_chain_and_layer_tree(&current_scroll_chain,
                                                 active_tree());
   scroll_state->DistributeToScrollChainDescendant();
 }
 
 InputHandlerScrollResult LayerTreeHostImpl::ScrollBy(
     ScrollState* scroll_state) {
   DCHECK(scroll_state);
 
   TRACE_EVENT0("cc", "LayerTreeHostImpl::ScrollBy");
   if (!CurrentlyScrollingLayer())
@@ skipping 987 matching lines @@
       worker_context_visibility_ =
           worker_context->CacheController()->ClientBecameVisible();
     } else {
       worker_context->CacheController()->ClientBecameNotVisible(
           std::move(worker_context_visibility_));
     }
   }
 }
 
 }  // namespace cc